2.5D does have its own unique benefits though - smoother animation, ability to vary camera angles for non-gameplay scenes like throws or super attacks, able to have much more characters for the same cost, able to have many costumes and items, able to have slow motion effects.
Also, due to low RAM and other hardware factors, HD sprites are actually more hardware intensive than 3D models these days. We're not likely to see a 1080p 2D game with as many frames of animation as 3S, for example.
I think the PS3 Naruto game (and other games) show that cel shaded models are approaching the attractiveness of sprites (not quite there yet though). At some point the artistic benefits of 2D will be overwhelmed by the benefits of 3D, and if we want to keep 2D gameplay this pretty much means 2.5D.
The only alternative is vector line-art based sprites, rather than bitmaps.