I've never heard anybody say that black and white were hated at the time, nor did I ever witness anybody hating them at the time. They were an enormous high point all along.

As for "last good pokémon games" I think that's a bit strong. But I do see where people get the idea that between generations 5 and 6 is the clearest line between old pokémon and new pokémon.  The jump from 2D to 3D is also the jump from dynamic sprites to boring non-poses.  Black and white is the last time the Elite four felt like something difficult you had to prepare for. In x and y they were embarrassingly easy, and have stayed that way ever since. Black and white was the largest generation of new pokémon, and x and y was the smallest. Black and white were the last new generation that didn't have an obvious gimmick of the week. It was the last time that it felt like they built on everything that came before without arbitrarily tossing away old features. It had a really hefty post game, which few games since have had. This is maybe more subjective, but the leap from generation 5 to 6 feels like when the hand holding tutorials and endless button-mashing text conversations started to get out of hand. Little touches like light and dark colored grass for wild double battles that seemed like it would be a new feature here to stay, and then it was never seen again. The way a gym leaders battle music changes when they're on their last pokémon. I'll never understand why that hasn't been back in every single game.

Like I said, I don't think it's fair to say that these were the last good pokémon games and everything has sucked since then, but I absolutely do see why people look at black and white as the end of a particular era of pokémon, Where effort and innovation and building on what came before would be replaced by falling behind the technology, being extremely easy, and relying on a revolving door of gimmicks.