For one, removing instance timer = making instances permanent is a surefire way to increase serverload as a malicious actor (bots creating instances of dungeons to such a degree that it will hog server resources), so that's why there is a duty timer & an AFK timer.

Normal dungeons and variant dungeons typically take no longer than 20min... you can't play 20min at a time without going afk? As for deep dungeons, part of the challenge of deep dungeons on the later floors "is" the time constriction, where you have to make careful decisionmaking on whether you spent Pomanders or avoid certain rooms. So... no?