People who only play one job act like their job can't still do other things.

If your only job is tank, you can still DPS.
If your only job is healer, you can still DPS.
If your only job is DPS, good dedication I guess. Hope you're really good at your job.

There is absolutely nothing wrong with the INTENT of OP's proposal: to allow job changing in a new kind of dungeon to encourage optimization. People will of course twist it to bash people who don't have the drive to play more than one job or learn enough about their one job to do more than one thing at a time. That doesn't make the idea inherently flawed.

Caring about hurt feelings is going to kill so many prospective ideas.