I truly don't get the power fantasy-necessity though. Actually I dislike that part about many isekai. When you're thrown in an unknown world, you naturally are weak, helpless and desperate. Slowly pulling yourself out of that situation is part of the appeal.

That's what non-isekai fantasy-anime actually do. See Berserk, Lodoss, and actually most anime. Guck, most STORIES are like that. It's just isekai that likes to completely ignore it. Well, not completely, it's what made ReZero interesting - although you could say they downplayed the act of dying over the course of the show.

Anyway, I for one don't watch anime or play games for a power fantasy. I play and warch for a 'what if' fantasy. Seems like the west/hollywood is better at that. Stuff like Harry Potter or Narnia do a good job at introducing another world where the heroes start out helpless, but then slowly find their place. That's what an isekai should be like. Imo.