Not saying I disagree with you but I think it's wanting decent post game exploration more than anything. Gen II had Kanto, Gen III (FRLG) had Sevii Islands, Gen IV had the Battle Zone, Gen V had almost a quarter of the map be post game, Gen VI had... a poor excuse for a Safari Zone, and Gen VII had a large empty area. So there have been things like the Looker quests in the more recent games but there have been no new areas to explore once you beat the E4 and exploration is one of the most important parts of in game Pokemon. We had great post game content for five generations but now the focus is clearly on VGC and online play. Even post game battling has been reduced back to a generic Battle Tower compared to the Battle Frontiers or PWT.*Sigh* This isn't as much about the franchise itself but more about the fanbase; a certain thing that often appears on wishlists that, at least to me, doesn't seem to take into account why it happened the one time that it did and hasn't happened since: wanting to visit a previous region.
I guarantee you, if you watch/read a batch wishlists from Pokemon fans at least half if not more will have visiting a previous region be on there. Now, this alright as a wish that are in general/something they would want to see in a future game... but it really annoys me when I see people wishing for this even though we've learned a batch about a new region and it should be very obvious we won't be visiting a previous region.
It all starts, of course, with Gen II. Once you got done with Johto, surprise, you could visit Kanto. It was a nice bonus, but there was reasons for this. Kanto & Johto are next to each other and share a Pokemon League, their stories and characters have direct connections, and Gen II was originally called Pokemon 2 so was intended to be a direct sequel. Since the games were running off the same engine there was little extra work they needed to do aside compressing it enough to fit both regions on the cart (and in the case of HGSS, they were working off the originals so didn't have to come up with all the changes that would have happened over the years).
So let's move onto Gen III, based on a region that's far enough away from Kanto & Johto that what happened in those regions wouldn't affect Hoenn and vice versa. Still, people got upset they couldn't visit the previous regions. Rinse and repeat for Gens 4, 5, 6, and 7. Aside from the remakes, I think it's fairly clear the situation with Gen II was a special one.
Now I'm not saying this can't happen, but if it were to happen again the previous region would have to be close by and there would have to be some kind of story connection for the player to decide to visit it (that's why the Unova player just doesn't hop a plane and fly to Sinnoh: why would they?). The previous region (and all its changes) would need to be part of the newer region's story in order to justify spending precious development time they could have instead put in the new region to make it better in places. But no, all players who wish to visit a previous region in a new gen just seem to want to do so for the sake of it.
Honestly I think the reason many people put visiting other regions in their wish lists is because it's far easier to imagine exploring them rather than a new area such as the Sevii Islands. I'm pretty sure that if it were a choice between exploring an old region or a new one, most would pick a new area.