We all want upgrades! On most positions.
Ok, so we upgrade Dier with player X, who is fractionally better on some aspects, equal on other, and fractionally worse on others. That still leaves us with liabilities in the positions behind him. Main problem as I see it is the lack of quality in depth. It has several knock-on effects, mainly that our first string players _cannot_ be rested as much as they should, "rest" usually means they are already injured. Secondly, lack of _real_ competition is a weakness.
I'm certain that had Dier (and Romero, for that matter) had a genuine competition for the position it would do them good.
Replacing Dier now, without fixing that, only shifts the problem a notch instead of solving it.
So unless we can (by magic, hypnosis, or plain witchcraft from Conte) make Davinson Sanches a consistent, safe and thrustworthy player, we need to start the replacement process in that end, not the other way around.
Sure, there are better defenders than Dier out there.
However, strip away those who do not want/need to move (ie. already at so-called "bigger" clubs and are first choice/regulars there) and those who are first choice at our peers, and you're left with those few who are either a slight upgrade, a talent or a gamble. The rest are just same sh*t, different wrapping, in my opinion.
The potential to upgrade our second string to someone who can genuinely challenge our first choice players are much better. If Dier should find such a challenge to his position to much to handle, and wants to leave, thats a different matter.