I don't think mpm is being insulting to ash as a character but i do think it is insulting to the fans of most of the post-os companions. I personally believe that they all deserve one last hurrah before the Ash saga ends.
People say this, but looking at the writing, I don't think they could've balanced the various companions (most who appeared in Journeys) and the returns of Ash's Pokemon at the same time.
Look at the Cilan episode for example.
Are you trying to be intentionally dense here? First of all, open endings themselves aren't a problem since a lot of great anime have them. Gon and Killua had one at the end of HxH, and Gintama's final scene is literally what MPM thinks it's gonna do, have the main protagonists run and say "our journey is going to continue". What makes all these endings work is that it feels satisfying to a long concluded and ardeous journey, something that can't be said for Ash. The first big mistake here was to promote 10 episodes of absolute nothingness as the "beginning of the end" of Ash's journey and have Gary ask that question in the opening trailer as though Ash was going to introspect, while he'll find the answer to that question in 5 mins anyway as we know. If MPM episode 11 was JN137, it wouldn't even have been this awful (still pretty trash but not as bad as it is now). That would've been underwhelming as you put it. MPM is insulting cause it was basically bait to hype the question of "what is the meaning of being a Pokémon Master" while having 10 padded out filler episodes of Ash doing things he's done for 26 years. This series never needed to exist.
If we want to talk about the distinction between "underwhelming" and "insulting", I gotta side with @SerGoldenhandtheJust
here. MPM is underwhelming, but ultimately it's just harmless slice of life adventures meant to be fun for the target demo of literal kids with some nostalgia sprinkled in for their parents or siblings. The equivalent of a post-game DLC.
I really don't get people acting so overdramatic about MPM while saying in the same breath it should've ended with JN136 or whatever. If we want to go there, really nothing in MPM is as "insulting" or spitting-in-the-face-of-the-people-who-stuck-by-the-show as Ash not using his reserves in PWC/Masters 8. Instead using his least developed team ever, with over half the members not even having the excuse of existing to shill the Gen 8 games.
For a series called Pokemon (aka Pocket Monsters) + all the talk about the bonds and friendships between trainers and their Pokemon, the ultimate irony is Ash's Pokemon got impacted the hardest. Sidelined all throughout Journeys, despite knowing it was the final season. At the very least, MPM features them (even then, not everyone got a return).
Hopefully the finale features appearances from everyone. Muk, Gliscor, and some others haven't shown up in promos yet.