Does not this sentence mean anything at all to you?
Absolutely not first because not all manuscripts are full manuscripts.The manuscripts that contain it are nearly as old as the manuscripts that don’t. So, argument from silence. You read a sentence and you think it’s the end of the world. This is called textual criticism where the absence of something in some manuscripts does not mean they are not authentic. There are much more criterias to determine authenticity, includind citations, translations, and other manuscripts.
In Favour of Mark 16:9-20
Codex Alexandrinus (A) - (5th c. uncial, Byzantine in Gospels)
Ephraemi Rescriptus (C) - (5th c. uncial, Alexandrian)
Codex Bezae Cantabrigiensis (D) - (5th/6th c. uncial, Western)
K (9th c. uncial, Byzantine)
W (5th c. uncial, generally thought to be Caesarean in Mark 5:31-16:20)
X (10th c. uncial, Alexandrian)
Delta (9th c. uncial, Alexandrian)
Theta (9th c. uncial, Caesarean)
Pi (9th c. uncial, Byzantine)
f1 and f13 (total of 16 Caesarean texts, 11th-14th c.)
28 (11th c. miniscule, Caesarean)
33 (9th c. miniscule, Alexandrian)
565 (9th c. miniscule, Caesarean)
700 (11th c. miniscule, Caesarean)
892 (9th c. miniscule, Alexandrian)
1010 (12th c. miniscule, Byzantine)
The Byzantine textual set
Some of the Greek lectionaries
and more importantly, we refer to the Church Fathers. Irenaeus quotes Mark 16:19 (~177 AD), and Tatian the Assyrian included the ending in his Diatesseron, (~175 AD). Tertullian refers to Mark 16:19 around 215 AD, and Tregelles reports that Hippolytus (~235 AD) quotes Mark 16:18-19 at least twice. In Cyprian’s account of the 7th Council of Carthage (256 AD), he records a strong allusion made by Vincentus of Thibarus to Mark 16:17-1818. The Apostolic Constitutions (3rd-4th c. AD) quote Mark 16:1619, as well as alluding to Mark 16:15 twice.
So should we believe the church fathers citations in addition to many manuscripts or arguments from silence?