Sanchez's has been worse than dier's. And the question isn't if either of them is a good defender. The question is who is a better defender. And you can't just ignore Dier's previous season with Conte and declare he doesn't fit in a low block.
A couple of matches here and there won't decide the overall progress and neither would tactical changes for the sake of the team undermine Ange's vision. If we were to go down to Dier or Sanchez(our only 2 senior defenders before Sanchez was off to Galata because Levy couldn't agree on a cober), we would already be in deep trouble. Then it's about making the best of a bad situation
If Ange deems them ready, fine by me. But if the choice is Dier or Sanchez, it's always Dier for me.
Sanchez is/was a better fit for Ange's system over Dier. Ange's system takes presidency over anything, I don't think he has ever played a low block and he even said that he would never alter his principles, so not only is this moot as Sanchez has gone, even if he were still here Dier wouldn't see a minute of pitch time over him.
Citing Dier's one good year out of how many bad ones only means that he was treading above water and last season he went back to his natural level.