Well, after gen 5, when they started using 3D models, things seem to have gone downhill.

For one, in the gen 6 games specifically, XY and ORAS, they changed the perfect level of difficulty the previous games had and made them ridiculously easy. And it's not like the previous games were even especially hard in the first place. You just had to put a little more effort into them to win. And the overpowered experience share didn't help make the game any less easy either.

Then the gen 7 games (SM) were more challenging, which was a good thing, but there were way too many cutscenes and handholding, and very little independent open exploration.

Then the gen 8 games (SS) were pretty much a complete mess, and arguably the worst games for their time. They brought back the gen 6 issues of being way too easy. There was the infamous bullshit dex cut. And not very much exciting exploration you could do. Not to mention, they story was pure crap.

That's not to say that the gen 6-8 games have no redeeming qualities at all, but their flaws are impossible to overlook, as they're just too obvious. So me, personally, I'd say they did indeed go downhill.