If you've ever searched for anything on Twitter, it's probably not going to be fun. There is a pretty common sentiment among Yankees fan, even up to the higher profile ones, that Mark Teixeira is not a "Good Yankee." Never mind the fact that he helped lead the Yankees to a World Series in 2009, but fans will tell you that it's "not worth it" if he's not going to be good for the entirety of the contract.
One of the more hilarious arguments is that Mark Teixeira is made of glass, has a baby wrist, or I'm sure there are plenty of other hateful and hyper-masculine diatribes out there. As we all know, Mark Teixeira underwent wrist surgery in 2013, and then spent most of 2014 trying to regain the strength he once had. It didn't really work, but a year away from the surgery probably bodes well for the strength of his wrist.
If all of these "Mark Teixeira is made of glass" arguments have weight to them, then we would find that first basemen of the same caliber would exhibit better health and performance over the same period of time. Well, they don't. Of course they don't, or else we wouldn't be here right now.
If one were to look at first basemen in the past forty years who produced more than 20 fWAR in their twenties, and then look at how they performed from age 29 to 34 (the course of Tex's contract with the Yankees), you can get an idea of how Mark Teixeira stacks up. Teixeira, for reference, put up 31.8 fWAR in his twenties with a 135 wRC+.
Players of this caliber put up an average 19.2 fWAR in this span, as well as 135 home runs in an average 768 games. Teixeira, by comparison, put up 16.4 fWAR and 160 home runs in 731 games. It isn't spectacular by any means, but to think that he was dreadful is just out of the question. He's less than a half of a standard deviation away from the mean in every category, so it's not like he's out of the ordinary or anything. Looking at some of the names ahead of him, you see some of the best first basemen of all time, including Jim Thome, Frank Thomas, Albert Pujols, Todd Helton, and Jeff Bagwell. It's unreasonable to think Teixeira should be that good, because what he has done is pretty remarkable anyway.
It's not Teixeira's responsibility to live up to that contract; there's nothing in there that requires him to meet some threshold performance to obtain that money, and it's not like he forced the Yankees to give him that contract. He got that contract, and he's done his best to play well.
Frankly, it's pretty funny how fans romanticize the performances of both Tino Martinez and Don Mattingly, even though Teixeira has been a better player from ages 29 to 34. I suppose it was because Mattingly was the face of the team and Martinez won all those championships, but Teixeira has been the better player in this age span. As Andrew pointed out, Teixeira is undoubtedly one of the better Yankees in history, and certainly one of their best first basemen. Criticize him all you want, but that's exactly what they paid him to be.