"Real time rendering" mostly means games, and performance is a very very easy bottleneck to hit for games. Because it is easy to make things more fun and interesting by scaling them up: more entities, more interactions, more options, etc.

So I believe you are quite wrong. It is trivial to scale-up a simulation to the point where you're at a performance bottleneck. The only reason we don't scale things up today is because our computers are far too slow to handle it.

And we stopped using Gouraud shading because it is horrendously ugly. We'd still be using it still if it looked half as good as rasterization does.