People made the seperation between sprites and 3D era, not only cause of visual style, but because of deepee changes to the gameplay itself.
Since XY, the games have more or less taken a more flash-in-the-pan approach. While being super impressive at the time, and fun on your first run-through, have painfully little to do when you're through with the experience.
I can't speak for everyone, but I loved going to battle facilities, unlocking events via strange methods, like the Regi trials in Gen 3, Contests in Gen 4, or even the World Tournament and Tower/Treehollow from BW2.
Hell, visiting a whole other region in HGSS felt like there would always be more.
XY on, however, it sorta stops at the champion fight and a singular battle tower. No other sizable post-game, lack of legends to hunt (aside from USUM), or any final locals. Instead, in the latest games, it's being sold back to people at an extended price.
It's understandable why people are upset over the changes. Especially when they come to expect more from a series as large as this.
