Average users will immediately notice the difference when their programs will not install. It does not take a genius to understand that it did not work with Windows RT and it would not work today, either.
Their programs will install just fine. If you haven't noticed MS is working double time to replace their old crap API stuff with new cross platform stuff.
https://www.theregister.co.uk/2018/10/12/microsoft_xlang/
For really old stuff, there is nothing stopping them make a translation layer as they did for yes Windows RT. Frankly do you really think at this point MS gives a shit about developers not using their approved windows store friendly APIs ?
MS wants a walled garden as much as Apple and Google... as long as they continue to run their old micro kernel they will never really have it. I have never suggested MS would move to a FOSS version of windows, their linuxed up verison of windows will be the most locked down version of windows they have made yet. All current software will run just fine MS snapped up Kenny Kerr and his WinRT... and they have him working on completely cross platform compilers for their libraries right now.