Why not create several pools and further decentralize the network?
Miners have the freedom to pick whichever pool they fancy. While the three you mentioned are certainly popular choices, they are by no means the only options available. In fact, you can even create your own pool from scratch, and it won't cost you a dime!
Now, about GPU mining. Truth be told, it hasn't been worth the hassle for ages. The electricity bills and the fact that graphics cards don't last forever make it a losing game. On the other hand, even those ancient ASICs are way more efficient than any GPU out there. So, if you're thinking about mining, go for an ASIC and leave the GPU behind.