I don't suppose you know who Agur is. I certainly didn't. Although the Proverbs are attributed to Solomon, the last two chapters are the Sayings of Agur and the Sayings of King Lemual.
In Proverbs 30:5-6 Agur says: Every word of God is flawless; he is a shield to those who take refuge in him. Do not add to his words, or he will rebuke you and prove you a liar.
Scripture is complete. I remember making a cup of tea for my parents when a small child. I thought I would experiment with flavors and added a small teaspoonful of mustard to each cup. It was not a success.
Some have deliberately added to the Bible. An example would be extra conditions for salvation such as the prayers of saints or the intercession of Mary, good works. The sects have extra books like the Book of Mormon or Science and Health with Key to the Scriptures. Other have eliminated part of Scripture. They don't have to be physically cut out pages, just disparage them. The creation? An ancient Hebrew myth disproved by Darwin. Jonah? A folk tale. The miracles? Sleight of hand or hypnotism. The resurrection? Meant to be interpreted spiritually.
Sometimes the alteration to Scripture has been accidental. An error of transplantion - it's not easy to get it completely right. Words change their meaning over time and even tracing etymology does not always help. How about copying errors? The famous "wicked" Bible had "Thou shalt commit adultery" and an early edition of the NIV had 'immoral' instead of 'immortal'.
Prior to 1947 it was assumed that many copying errors had crept into Scripture over the years like Chinese whispers, but the Dead Sea Scrolls have shown us that Scripture is remarkably free of mistakes. Over 1000 years from the DSC to the Masoretic text the Old Testament books were copied accurately.
So where does that leave the end of Mark? The earliest Manuscripts end at Mark 16:8. The rest of the chapter is written in a different style. It speaks of Mary Magdalene as if she were being introduced for the first time rather than having been a major character in the previous chapters. It is really a summary of what is found in the other gospels. But is also contains contentious promises that do not hold up: picking up snakes and drinking poison. I know that there are various groups in America that pick up snakes as part of their worship, and indeed you can immunize against snake venom so as to be unaffected, but even pastors in these churches are sometimes killed by the bites of rattlesnakes. I know St Paul was bitten by a snake without ill effect, but someone else was cured by Peter's shadow. It is dangerous to generalize from the particular if your only warrant is a passage added 400 years after the gospel was first written.
To my mind, the most compelling evidence for the later interpolation of Mark 16:9-20 is seen in the structure of the gospel.
Mark 1 to 6:29 starts and finishes with John the Baptist. Jesus is established as the Messiah.
Mark 6:30 to 8:21 is topped and tailed with the feeding miracles. The first Jewish, the second, gentile. The gospel is not just for the Jews but for all mankind
Mark 8:22 to 10:52 is contained within the healing of two blind men. The disciples have eyes but will not see.
Mark 11-13 begins with the cleansing of the Temple and ends with the prediction of the destruction of the Temple. Temple worship is to cease. The sacrifice of animals was but a picture of the sacrifice of Christ.
Mark 14 begins with Mary anointing Jesus and ends with her going to anoint his body and finding no body there. Jesus the anointed one; prophet priest and King.
There it is complete. There is no need for an extra coda. It is complete and flawless.