Obviously this is mostly about Anthony, but since you also brought an extension for Casas into it a couple of times, I think he's fair game too!
Casas might actually be more of a case for caution in these early extensions.
There's plenty to like about Casas's power and his patient, methodical approach at the plate.
But now for the reality check:
In 840 MLB PA's, Casas has an fWAR of 2.7, which makes him roughly an average player on a rate basis. His WAR gets hammered by poor defense and lack of speed - as it should.
The other issue of course is that Casas had a long and troubling stint on the IL due to an injury in his rib cage area. No one really knows if this is an injury that will continue to hinder his career.
For those reasons I think the Sox may have lucked out in Casas turning down their first extension offer, and he might not get a second. I like him because he's homegrown, he has a personality and he has the aforementioned power and excellent approach at the plate. But the hard cold reality is that the rest of his game is weak, and overall he's probably an average MLB first baseman with injury concerns. All of which just illustrates that it isn't always a slam dunk to tie up the kids, with some you're better off to get the cheap production while it lasts.