>>54290716XY can't really be put at fault for being mostly empty games since it relied mostly on older content being translated into 3D for the first time as a major selling point and the Mega Evolution mechanic, it was an experiment to see if mainline could handle the hardshift into 3D at all.
ORAS is kind of the moment where things start to get dicey, extra content being eliminated in favor of lukewarm rehashes, Emerald being turned into Delta Episode instead of the remakes being based on Emerald itself to begin with, killing the Battle Frontier and neutering Contests in favor of the newer audiences, Nu-Pokemon design began mostly here.
Gen VIII was hurt by the stupid rushed development cycle, instead of making USUM first and then using the Gen VII resources for making another interesting title or Sinnoh remakes they wasted a whole gen and it's engine on only Alola. And the worst part is that, USUM are actually great pokemon games, hardly masterpieces of course but their quality got overshadowed by releasing base Sun & Moon first and then pulling that "Ultra Changes" stunt later which in the end hardly changed anything, they're a superior version of Sun & Moon because it's LITERALLY Sun & Moon but better, not even worthy of calling them rehashes or anything like it because any substantial changes introduced hardly mattered (Ultra Recon Squad for example).
Sword & Shield took all of these problems, mixed them together and applied them on a macro-scale: Experimentally empty game for the newest chapter in the series' history with the jump to the Switch like somehow Game Freak would be unexperienced at making bigger games, promising substantial changes to the formula only to deliver worthless shit like that huge open area and worst of all, instead of poor post-game like the rest of Nu-Pokemon they actually just replaced it with PAID DLC. SWSH was like the tipping point of shittiness, even if we reach another rock-bottom nothing will come close to SWSH ever again.
