First you have to prove that level of detail=gameplay. Then you have to prove that the art direction cannot be reasonably duplicated with reduced detail.
I'm not a game developer but even Firaxis which is developing Civ:revolution mentions the fact one reason why Wii version was put on hold is the art work had to be completely redone (no ps2 port to use) and they didn't have the man power to do a completely different version. Rev doesn't have any where near as nice of graphics as DMC4.
And yes graphic can and does improve the whole experience of a game which is why PS /N64 isn't the current consoles sold in stores. Gameplay means nothing to a blind man. :)