I suppose I hardly noticed how long it took for DX12 to work because games had DX11 modes basically the entire time that Proton struggled with it. So again, not trying to sway you on anything, but optimistically, there's going to be little to no games pushing any kind of envelope when a new technology like that comes along. It's already prohibitively expensive for games to do so today, such that there are very few good games in a given year that make use of the latest tech.

Appx and UWP are more packaging (like rpm, deb, msi) formats than an executable format. PE is behind everything in windows and the amount of effort required to change that seems truly Herculean, even for an organization the size of Microsoft. Heck, even executables for windows ARM were PE (as well as UEFI binaries). But even if you assume that they do go forward with something that insane, if they want developers to write software for it, they'll have to publish the format specs so that gcc and llvm can implement them so that the tens of thousands of existing libraries could be ported