This is a very low-quality argument. Game developers quite clearly cannot only target DX12 Ultimate and have their product only work for a small percentage of users on PC.

If you want to target PC, you have to deal with hardware fragmentation. You can pretend this is a Vulkan issue, but it's just a reality of the platform.

This is not feature discovery, though - this is feature constraining, which is rather different, and doesn't work at all if you want to get a game out to a large audience while also taking advantage of different hardware configurations.