Why? The current limits have a benefit - they encourage people to actually concentrate on their models and be as efficient as possible.
On the other hand, you could say that they simply force people to create inferior models. It's like forcing someone to use the Quake 2 engine for a game instead of the Quake 3 engine. So what if he optimizes his models and levels more. It still looks worse, and both easily run fast enough on an average PC.
Besides, if the source-code is really brought up-to-date graphics-wise, the speed increase will be at least enough to triple the poly-counts on everything and get the same FPS.