It's not an opinion, it is an absolute fact and there is more than enough data on Sarkisian.
Sarkisian went 5-7 at Texas. He had a historic 6 game losing streak, a streak that hasn't happened to any coach at Texas since 1956.
He lost to Kansas at home.
Those are all facts - no opinion whatsoever.
Sarkisian's career conference record is 34-33. By comparison Tom Herman is 34-17, Charlie Strong is 43-37, and Mack Brown is 157-87-1.
Sarkisian's single best season as a HC is a 9-3 year where took over a loaded roster recruited by a monster recruiting staff (Kiffin/Ogeron/Tee Martin/Clay Helton, etc) at USC. That USC team didn't require a scheme change (they used the same exact verbiage and scheme) or a talent upgrade, and yet he finished 4th in the Pac 12 South and 5th in the conference overall, that's his peak in 7 years as a HC. He's finished ranked only twice in his seven year career (at 20 and 25) and only coached 1 bowl game where he won by a FG over a Nebraska team coached by Barney Cotton. By comparison Herman has finished ranked 4 out of his 6 seasons and is 6-0 in bowl games with 2 top 10 finishes, Charlie finished ranked 3 out of 10 seasons and coached bowls in 8 of his 10 years, and Mack finished ranked in 19 of his 33 years including a National Title and 6 top 5 finishes and way too many bowl games to count.
There's not a single opinion here but there is more than enough data to call Sarkisian an abject failure this season and more than enough to determine that he is exactly who he has been and that who he has been is not good enough for Texas. How much more data do you need? Do you need him to "get his guys in" like he did at Washington? Let's take a look at that data - in year 4 and 5 at Washington when he "got his guys in" he was 5-4 in conference both seasons, and was blown out multiple times.