But, see, that's what I don't get. Why do they need the franchise to "grow up" with them? What even is Power Rangers without Bulk & Skull and the other cheesy elements? It's just any other mecha-action product. It's fine to take your fandom seriously, but you should recognize that the fandom is for something that has those elements in it. Star Wars fans are the absolute WORST about this. Power Rangers is less serious than The Avengers are, but in an ideal world, BARELY.
When they made the new MOTU series, one thing the co-writer told Kevin Smith was, "Don't give in to the urge to make fun of the original show - treat it like Shakespeare." Yeah, they did age it up some with some extra violence (too much if you ask me), but they also embraced the sillier elements, like the Bashasaurus vehicle and othe crazy toys that were shown in that world. They didn't go so far as to break the universe, especially since the writing was pretty basic on it. TMNT is a little different in that they started as a very gritty comic book - of course, that's not what got popular and now it is what it is.
On the topic of franchises outside of Beetleborgs:
A property should be allowed to be changed over time, and remaining static doesn't always guarantee success. Look at Batman, wide open for interpretation. Mention the new Pattinson Bat movie and I'll mention the new DC League of Super-Pets. It's not all dark.
Franchises that have been around for a long time tend to gather invested fans.
Being invested in the characters means the fans may very well want to see them progress beyond just a superficial, cartoony presence.
Fanfiction is a thriving practice because of all this.
Ask Buzz, he's invested in the outcome of the Pirates series, so he could easily explain his POV on these matters.
Fans love and relate to these popular franchise characters for a long span of time.
Flabber is a major exception to this.
(And on the opposite end of the spectrum, some franchises benefit from being redone as comedies. Dragnet, Starsky & Hutch, 21 Jump Street, etc. No big outcry with those changes. Not like the MOTU fans griping about He-Man taking a backseat to Teela.)
“Mature” or “serious” does not always equal ultra-violence and sexual situations, as opposed to say, deeper character development, broader story progression, and/or bigger themes and how the characters are relative to those themes.
Then there's shows like BEETLEBORGS...
There’s not going to be a Joaquim Phoenix Flabber to bait the Emmies or Oscars. If a new series features adult Beetleborgs, you still have to look at the source material and see that a mature change to the show means a complete overhaul: no Flabber, no goofy monsters.
The series was a cheap cash-in that's entertaining for it's flaws and goofiness. The characters are one-dimensional. If ever a remake should happen, it would work best as a comedy/satire. People are going to be high watching it anyway.