I've been wondering the same thing. We hear about games "knocking off" tf2 or Minecraft, but those seem like natural eventualities to me. I love my aaa games and their beautiful assets, but even the prettiest asset gets boring after 1000+ hours of multiplayer. That's where a Minecraft-esque voxel-based system allows for a simple framework for the community to make tons of new content to keep things fresh AND actually have fun creating it. It's not "work" to make most kinds of functional assets in a voxel-based environment, so you don't even need to pay them to create content for your game! Likewise, the tf2 route is more of a compromise where the art style makes it easier to create content, but you do need to provide some means of compensation to content creators since some "unfun" effort is required. However, this can actually have it's own advantages! In the case of tf2, you can let content creators sell straight to users in a free-ish market where the best content should naturally rise to the top. I think voxel-based games are still superior for modern mass market multiplayer games, but there are definitely some options out there.