They could, of course, add a timer. The shield around the tadpole only lasts so long until it dissipates and you die horribly, story over. Having a time limit would be just as effective at forcing people to limit their gameplay as a fatigue system. However, I suspect timers are disliked by mainstream audiences even more than fatigue.
I would say they are disliked by most people (see pathfinder which is definitely not a mainstream game)
Oh I know. I enjoyed them in Pathfinder actually, although I felt they could have been stricter with them. To each his own though, I agree I didn't need to specify there.