Plenty of bollocks here that I won't even quote.
Magical Girl-ing here is pretty clearly a stand-in for the (not always, but dominantly) female experience of having a caretaking role thrust upon you and being expected to selflessly fulfill it until you're such a burnt-out husk you can't even imagine doing anything else. Childhood is the stage where you're still find yourself, and the regression here therefore representative of an opportunity to define yourself anew after being denied an identity for so long.
And even if we pretend theme and allegory don't exist or matter, I still fail to see how anything done here is creepy. They are, after all, not the sort of men in urgent need of castration. By that same logic I could pull out a mirror and ask you what sort of perverted looser consumes magical girl media well into adulthood; that's kinda sus, init?
Stop trying to read secret depravity into something moving and absolutely wholesome. Go write some "[Childhood hero] is actually in purgatory" creepypastas instead or smth.
If they HAD been trans, it would have been heartwarming. Fighting for humanity for decades so they could have the body they were meant to have?
That's heartbreaking, and touching, and bittersweetly sad.
But roleplaying as kids?
If this was an allegory for trans-hood, it was a bad one, cause it's dipping into an area of fiction that IS commonly presented as uncanny.
Benjamin Button. Marvel comic book villains like Topher and Abigail from the Runaways (sorry, that's a deep cut). A villain in the book "The Rook" (won't name because it's a spoiler and that's a great read).
Also, this still is an issue that isn't resolved: Makoto CHOSE to hide her real identity, and used her once-in-a-lifetime wish to become a child to spend time with Sayakawa. Without consulting her, without knowing her, without even having the kind of relationship where Sayakawa would tell Makoto she was retiring. They were not close. Makoto approached Sayakawa under false pretences, and DID NOT TELL HER the life-changing decision she made, just to possibly maybe get close to Sayakawa, and thus the underlying intense pressure behind their interactions.
Sayakawa CHOSE to become a child, under the (incorrect) assumption that this girl that was visiting her was a normal girl, putting a HUGE burden of responsibility and TIME on this random child: that is a lifetime's worth of obligation. Haunting.
It did work out.
But, and this is definitely my personal bias at this point: I think there would have been a beauty and a grace in their aging together, and meeting as adults, rather than reverting and trying to go back and experiencing a second, false childhood.