As far as I've read, there's been heavy hints that Team Rocket is not post-game, and it's actually part of the actual storyline.
Can you source those hints? As far as I've seen, the only official recognition of anything related to Team Rocket are those grunt clips from the cards that Serebii posted. I may have missed something, and I don't go out of my way to dig around in supposed leaks or rumors, but at the moment, it seems like the internet will be absolutely rife with Team Rocket rumors, so everything should be taken with more than just a pinch of salt. At this point, trying to speculate on Team Rocket's role in the story is like trying to say that the Switch game will be set in a region based on the UK. Just as we only factually know that they're working on a Switch game, we only factually know that Team Rocket will be involved in some way with USUM, and we know literally nothing else.
Because it means they're making new versions of Sun/Moon only with added stuff that comes from Gen 1 when they could use that to expand further on Sun/Moon stuff instead.
Or, ya know, expand Gen 3-6 stuff for that matter. It doesn't need to be Kanto every single time.
Of all of the past teams, Team Rocket was really the only group that was positioned to come back in as a whole. Plasma's story is too big and, along with Flare's, is completely unrelated and would be overshadowing to try to strong arm either of them in. Colress and Charon could work well enough on their own, but neither necessarily represent their teams' overall goals.
Rocket, Magma, Aqua, and Galactic all collapsed when their leadership left, either because Maxie and Archie were overwhelmed, or because Cyrus straight up abandoned Galactic to stay in an alternate dimension devoid of life. Only Giovanni came anywhere close to stating the intention to rebuild. If they were going to bring in an old team for whatever reason, and have them fit into the new plot, which team do you pick, if not Rocket? Every group except for Rocket and Skull had a fixed and rigid purpose or plot, whereas the former two are rather flexible in where and how they can be used. Rocket works as a mafia organization that can slide in anywhere, and of any of the groups, they are definitely the most opportunistic.
This isn't Gen I pandering, not to the extent that locking Alolan forms to Gen I pokémon exclusively, using the Kanto starters in X and Y, and copying Santalune Forest as a Viridian Forest ripoff was, because Team Rocket is already a multi-generational organization. At the very least, of all of the other things that actually
is pandering, this is probably one of the things that is the least worthy of complaints and accusations. Team Rocket's return was predictable, if not here, then as some point in the future, because it was something that was never really off the table.
For all that we know at this point, this actually
could be some kind of "Rainbow Rocket" amalgamation of other teams, so there's not really any point in yay-or-nay bandwagoning when we really only have one picture to go from. What we do know is that this is
not the same Sun and Moon like so many wanted to believe, and that the story is getting deeper than we already thought it would, and I, for one, am grateful for that.