In time, everything can become comfortable. That’s how bad habits develop. Once they develop, a new routine may not be beneficial.
You should pick your battles. Is the mechanic something that is actually detrimental to the player? Is it actually impacting their performance? Or is it just that the coach was taught one way and only knows one way?
Many “eccentricities” or “bad habits” are things that players use as timing mechanisms or for mental preparedness/fighting nervousness. Before you decide that you needto “fix that”, ask yourself a few questions:
1) What is the harm?
2) Why do you think your way is better?
3) Can you explain AND show the player the benefit?
4) Are you able to accept it if you are wrong?
Essentially, make sure any changes are about the player, not you.
There are different degrees and definitions of comfortable...
For example, player favors a no-stride approach, coach forces them to narrow their stance and stride. That approach is not very comfortable for the player and causes them issues. Is a stride necessary? No. Therefore this is a case where a coach is making the player unnecessarily uncomfortable. I'm talking about game swing, there are some drills that are easier with a stride.. and vise versa as well
On the other hand, player has some technical flaw in their swing that is causing or well cause issues as pitching gets better. Coach addresses it and the hitter feels uncomfortable with the new change. In this case the player needs to buy in a little more and understand that this discomfort will pay off in the end. Coach needs to stress that the change may feel a little uncomfortable in the beginning but should subside. If it doesn't he/she will try another approach with her.