This is true for simple shaded objects. But if you use expensive shaders and many dynamic lights then it is the other way around. It is better to create many smaller objects instead of one big and to let the engine do a good culling to calculate only the needed lighting, shadows and shaders.

Polygons do not bring a modern GPU down, but fragment programs (shaders) do and modern games use them very intense. So a good scene and light management is still needed, maybe even more than before.

And because of that optimized engines like C4 will be able to run more detailed and better illuminated/shaded geometry than other ones, especially indoor. There you can use zones and portals to render only what you see on screen. And there is still some visibility culling and intelligent culling of shadows even through portals. This is not trivial.

I miss something like that in the feature list of Unity. So it might be a good alternative for casual games but not for beauty indoor games with bigger levels. Though they proved that they can render outdoor very good with LOD and streaming of terrain chunks.