They already are. But the future will probably not look like this if the current trend continues. It's just not efficient enough when you look at the whole design process.
As I said twice now already, efficiency is not just a question of rendering pixels. When you take the entire development lifecycle into account, there is a vast opportunity for improvement. This path is an obvious continuation of current trends we see today: Why spend time optimising when you can slap on DLSS? Why spend time adjusting countless lights when you can use real time GI? Why spend time making LODs when you can slap on Nanite? In the future people will ask "Why spend time modelling polygons at all when you can get them for free?"
Nobody will spend time modelling polygons. They will convert gaussian splats to polygons automatically, and the application will rasterise polygons. This is how it's already done, if we went back to ray marching NeRFs we would be going backwards and would be an incredible waste of performance. Polygons are here to stay for the next 20 years.
