You'd be surprised by the size of PR's adult fanbase. The show might be marketed primarily at young kids, but some of the sesaons also, tonally, appeal to an older audience, most of which watched the series as kids and has stayed with it - to some degree - over the years.
Not to mention those of us who started watching as adults because the female Rangers were just plain hot. Especially Amy Jo Johnson, the original Pink Ranger. She's probably more responsible than anyone else for giving the show its initial popularity among adult male viewers.
BTW, to the people who may have given up on the franchise after its early seasons, you should give it another chance and check out the later seasons, in particular the following:
Power Rangers in Space
Power Rangers Lost Galaxy
Power Rangers Time Force
Power Rangers Dino Thunder (not personally a big fan, but enough of the fandom likes it for me to reccomend watching it)
Power Rangers SPD (again, not a big fan, but it's popular with the majority so I'm again recommending it)
Power Rangers RPM
I didn't care for SPD, though maybe that's just me. It did go for a more sophisticated approach, with a lot of tension among the Ranger characters, but I just didn't care for the characters or their portrayers that year. Otherwise, I agree with your recommendations, although I'd add Mystic Force
and Jungle Fury
from the Disney years.
I did a 3-part overview of the Power Rangers
saga to date on my blog, beginning here: