The 10% were not just in context of "switching" the API but in general the maximum performance gains that are possible (other then special cases like Multi GPU).  DX12 is not really "better" it just gives the developer more control and lets the driver do less.

DX12 was hyped up to get people over to Windows 10.

Believing in marketing gets you nowhere, it's made to seem better than it really is on purpose to make you want it more. Considering the privacy risks when using Windows 10 which are unavoidable, and seeing people use it anyway is just so revealing of how stupid people are in general, and not everyone upgraded purely for DX12 like gamers did, people are just misinformed and act out blindly.

DX12 is a failure, not because it can't be good, but because it was pushed out too quickly and is restricted to a single useless (at this point in time) platform called Windows 10.

Correct me if I'm wrong...But isn't the 10% perf gain only relavent to games that were dx11 being switched to dx12. Building a game from the ground up on dx12 is much better, right? Although I don't know why anyone would use it since Vulken is starting to shine.