the reason you can play so many builds is because D2 is easy. Even the Endgame is easy. The difference with D3 is, that the endgame can be as hard and demanding as you want (unlimited greater rift level) so the most optimal builds are the cookie cutter ones. In Diablo2 it is quickly possible to overpower the endgame.
the only thing where I agree with you is that itemization is difficult. But only if you play melee. In this case you need a decent weapon.
Well, at normal and nightmare we can have an easy time, i agree with you. But at hell mode the game really starts. At this point, time is the main factor to define any build at D2. I have some chars at lv 99 and no end build at all yet.
At D3 it is too much easy to find the itens and prepare an end game build. At some point, any progression is totally based at paragom level and no itemization at all. I miss this at D3. At D2, itemization really makes difference, but as you said, it is useless once you can kill everything at hell mode. D3 lacks versatility. D2 lacks an endgame meta now.
Edit: I dont know, but if the devs would bring the rifts and great rifts to D2, probably i would stop playing D3 at all now.
I'd be more likely to suggest that a Grindy Beat-Em-Up RPG (or whatever you end up wanting to call Diablo games, Action RPGS too sometimes) are the culprit in themselves. You cannot derive joy from playing a game like this for too long. You can't create complexity without ruining the simplicity of the beat-em-up style. People want to smash through things, but ask for a challenge. Regardless of what you add to these games, it will always be lacking "something" after a bit, and you'll forever be free to fill that "something" with what fills your imagination I suppose.
But, a few alterations or additions should be enough to sooth individuals over for small periods of time.
After all, it's just a game.