To me, the big sign of a bad trend when it comes to excessive pandering to gen 1 fans is not the XY megas or the Alolan forms (that was fine, in fact I liked it), or even the Let's Go games (though those are pretty excessive). It's the fact that there's so many spin-off games, some on the older/newer systems and some on mobile, and yet none of them have ever been developed directly by Game Freak -- until now.
Pokemon Quest, starring exclusively Gen 1 mons only. With the chance for making something unique in this spinoff, Game Freak decided to ignore six generations of creatures, 650 of them, to only go with the first ones. And considering they went on to make Let's Go shortly after, makes this a baffling and worrying trend.
These sort of forum topics don't help either.