I think Dier has been above average compared to his peers this season and more joyously well above his own performances from the last 2/3 seasons (while acknowledging the mitigating circumstances during those seasons).
Previously he was still in the early stages of reinventing himself as a full time CB, so I'd cut him some slack as he settles into a role that he is essentially learning. Now we play in a 3 the CCB spot suits him, and it's only since Conte arrived he's had chance to play it, and since then I can't really find fault in most of his play/performances.
There are probably better players out there to play the role, but I guess they'll cost £50/60m, so that won't be happening. But there is also certainly no-one better at the club to play that role, so ruthless assessment or not, he'll be starting.