We are just seeing the natural evolution of gaming as the technology gets better. From single screens (asteroids, berzerk, pac-man) to side scrollers (defender, moon patrol), to plane scrolling (xevious), to forced perspective (space harrier), to 2.5d like wolfenstein to modern 3d.
That doesn't mean the old tech will be abandoned, just that it will be more choices overall, even though there are less in any particular area. And as the technology gets better and better there is less of a development effort to make games in the older technology area. You can bang out a space invaders clone in Flash in about an hour if you know what you are doing. I'm sure the original assembly development took months.
I think Planescape was a bad example, because what made Planescape glorious wasn't the graphics or gameplay, which were pretty dated by the release date, but the unrivaled story. And part of the power of that game was the graphics helped the atmosphere and immersion rather than distracted from it.
2d is far from dead. World of Goo, Braid, Shadow Complex, Mega Man 9, Trine, Castlevania:OoE, Bionic Commando ReArmed are all recent, well reviewed games. How much more do you need? The background in all but Mega Man and Braid are 3d but the gameplay is all on the 2d plane.