I find those points of theirs to be pretty weak.

1. With Flatpak most of the runtime stuff is packaged separately, so on Flathub for example you have most of the applications using common runtime components. Sometimes you need multiple versions of one of them, but the same goes for a lot of Microsoft’s stuff.
2. I don’t find Windows to be any more backwards compatible than Linux is. On Linux you get the advantage of having access to the source more often, so source is going to be compatible for longer than binaries will. Source can be updated for compatibility too, giving it infinite life. (Of course, binaries could have infinite life if you take into account all the compatibility methods like virtualization.)
3. Yeah I’m sure business just love using ancient proprietary software and not being able to upgrade their systems. Oh wait, I thought compatibility was strong on Windows. (Not that all such software runs on Windows, but of course a major chunk of it does.) Businesses will eventually have to replace that old stuff, whether it’s sooner or later. It would have been better if they had the source code in the first place and could use that to help them upgrade or replace it.