Mark 16:9-20 is not original, it is corrupted.
The famous “Longer Ending of Mark” is the single longest textual variant in the New Testament. Most contemporary textual critics would reject Mark 16:9-20 to be original:
The passage is not found in א or B (two of the earliest complete texts of the NT in Greek dating to around the 4th Century). One other 12th century minuscule referred to as 304 omits the longer ending. Besides these three Greek witnesses, the longer ending is contained in every other Greek manuscript of Mark.
The Syriac Sinaiticus (~5th Century ) omits the longer ending. Some Georgian and Armenian translations omit it as well.
Codex Bobiensis (5th Century Latin Manuscript) has an alternative shorter ending right after verse 8 of just one verse and doesn’t have 9-20. This manuscript also contains a unique read at Mark 16:3.
A few Old Church Slavonic mss (probably the earliest ones being 10th or 11th century) only have verses 9-11 as opposed to 9-20.
Many manuscripts contain this shorter ending AND the longer ending (L, 099, 579, 1602, 083, ψ).
Codex Washingtonianus (4th or 5th century) is unique in adding a lengthy passage in between verses 14 and 15.
Jerome notes there were manuscripts that did not contain the passage:
Some manuscripts contain asterisks and other critical marks next to the passage indicating the scribe knew of a variant in the passage (like 205).
In conclusion, despite Mark 16:9-20 being in the majority text, it tends to still be deemed unreliable. Our two earliest witnesses that contain Mark 16 omit these verses, we see the passage appearing in various lengths, we see shorter endings appearing in the manuscript tradition, we see Jerome noting some passages did not contain the longer ending, and some scribes indicating they knew the passage was spurious. The passage has just been so susceptible and so variant driven that is extremely unlikely it was original to Mark’s autograph.
There is also a marginal note in one MS (an Armenian version) that states that Mark 16:9-20 was written by “Elder Aristion”. This was probably the elder Aristion known to Papias (who wrote c. 110-120 CE) from whom he received oral traditions about Jesus.
I find the external (MSS) evidence compelling, as explained above. Metzger (Textual Commentary on the NT) also adds the weight of the internal evidence:
The longer ending, though current in a variety of witnesses, some of them ancient, must also be judged by internal evidence to be secondary. (a) The vocabulary and style of verses 9-20 are non-Markan (e.g. απιστέω, βλἀπτω, βεβαιόω, ἐπακολουθέω, θεάομαι, μετὰ ταῦτα, πορεύομαι, συνεργέω, ὕστερον are found nowhere else in Mark; and θανάσιμον and τοῖς μετ’ αὐτοῦ γενομένοις, as designations of the disciples, occur only here in the New Testament). (b) The connection between ver. 8 and verses 9-20 is so awkward that it is difficult to believe that the evangelist intended the section to be a continuation of the Gospel. Thus, the subject of ver. 8 is the women, whereas Jesus is the presumed subject in ver. 9; in ver. 9 Mary Magdalene is identified even though she has been mentioned only a few lines before (15.47 and 16.1); the other women of verses 1-8 are now forgotten; the use of ἀναστὰς δέ and the position of πρῶτον are appropriate at the beginning of a comprehensive narrative, but they are ill-suited in a continuation of verses 1-8. In short, all these features indicate that the section was added by someone who knew a form of Mark that ended abruptly with ver. 8 and who wished to supply a more appropriate conclusion. In view of the inconcinnities between verses 1-8 and 9-20, it is unlikely that the long ending was composed ad hoc to fill up an obvious gap; it is more likely that the section was excerpted from another document, dating perhaps from the first half of the second century.
It is nearly unanimous that Mark 16:9-20 is a later addition. Textual evidence counts heavily against the position that it was originally in the Gospel. There are also lexical differences. N.T. Wright says:
The problem is well known. Stated simply (those in search of the full complexity can find it in the critical commentaries and monographs) it appears like this.5 The earliest manuscripts of the gospel, the great fourth-century codices Sinaiticus and Vaticanus, conclude with 16:8. They are followed by several later manuscripts, and some of the early Fathers of the church either show no knowledge of the longer ending or show, even while reproducing it, that they know it to be dubious. (Unfortunately, none of the many earlier papyrus fragments of New Testament material contains Mark 16; we can always hope for a providential accident of archaeology.) But the great fifth-century manuscripts, led by Alexandrinus, include the ‘longer ending’ (verses 9–20), and most subsequent manuscripts follow this lead.

In addition, four manuscripts from the seventh, eighth and ninth centuries, and some later ones, insert the so-called ‘shorter ending’, in effect verse 8b; and all except one of these then continues with the ‘longer ending’ as well. A good many of the manuscripts that do contain the longer ending, however, have marks in the margin (asterisks or obeli) to indicate that the passage is regarded as of doubtful authenticity.
The apparently independent omission in the two fourth-century manuscripts, coupled with all the other scattered evidence, makes it highly likely that the longer ending is not original. In addition, though the content of verses 9–20 contains some apparently Markan features (e.g. the disciples’ lack of faith in 16:11, 13, 14), in other ways it looks suspiciously as though it is derived from elements of the resurrection accounts in the other gospels.6 Thus, for instance, 16:12–13 is an obvious summary of Luke’s Emmaus Road story (24:13–35); the appearance to the disciples as they were eating (verse 14) belongs with Luke 24:36–43; the commission in verse 15 is parallel to Matthew 28:18–20; and the ascension in verse 19 is taken from Luke 24:50 and Acts 1:9–11. And, as is often pointed out, the command about the necessity of baptism for salvation (verse 16) and the the list of wonderful deeds the apostles will do (verses 17–18) look as though they are a summary of some aspects of later church life.7 All of these have led the great majority of contemporary commentators, of all shades of opinion, to agree that, though the longer and shorter endings are extremely interesting, they are almost certainly not by Mark.

N.T. Wright, Resurrection (2003)
As this article by F. C. Conybeare shows, the Longer Ending to Mark (Mark 16:9-20) is attributed to Ariston the Elder in one Armenian MS. What is interesting here is that Papias of Hierapolis (who wrote c. 110-120 CE), who passed on the tradition that the gospel of Mark derived from the preaching of apostle Peter, named an elder Aristion as a key source of his information. Also one of the stories that Papias mentions, that Justus Barsabbas drank snake venom with no ill effect, is apparently alluded to in Mark 16:18. There is plenty of evidence that the author of Mark was trying to discredit the disciples, and this is widely recognized in scholarship. The disciples are always portrayed as dense and uncomprehending of Jesus’ teachings. He is betrayed by Judas, then deserted by the others after his arrest and denied by Peter. Only demons and Gentiles, women and marginalized individuals such as lepers recognize Jesus as the son of God, not the disciples or most Jews in general (not even Jesus’s own family, including his mother, who thinks he is crazy, which would certainly indicate that Mark never had any kind of virgin birth story). The only disciple to call Jesus “Messiah” is Peter, but then even Peter misunderstands what the Messiah is supposed to do and ultimately abandons Jesus when he is arrested.

There is also a thematic undercurrent throughout Mark in which Gentiles repeatedly have more faith than Jews (e.g. the Centurion, the Syrophoenician woman) and the Parable of the Tenants appears to warn that something will be taken from the Jews and given to the Gentiles (Mk. 12:9). Mark’s ending at 16:8 makes perfect sense as a revelation to the Gospel’s Gentile audience that they have now become the heirs to this knowledge and this faith. this especially would make sense in light of the recent (relative to the authorship of Mark) destruction of Jerusalem and the Temple by the Romans (alluded to by the Gospel in both the Parable of the Tenants and the cursing of the fig tree).
https://www.amazon.com/Mark-Traditions-Theodore-Weeden-Sr/dp/0800613716