It only matters if you don't have the source to your programs. So yes, there is a huge corpus of programs where this matters. But there is also a large library of programs where the source is available and backwards compatibility does not matter nearly as much.
As a concrete example, the source to quake is available, this has allowed quake to run on so many platforms and windows infamous backwards compatibility has little effect in keeping quake running, windows could have broken backwards compatibility and quake would still run on it.
if you have the source to those programs, and are willing to (sometimes significantly) rewrite parts of them and recompile (see: wayland, for example)
The amazing part is that you don't need to do this in windows whether you have the source or not. I am a linux user, but for all their faults, Microsoft got their backwards compatibility stuff right. Something that the oss world, on average, needs to be convinced it's a desirable thing.
SDL was born from Icculus to run commercial games without issues on X/GL or whatever.
So, it's actually a NOT in your clause. More like a 99% of the existing graphical games modulo some oddies with Ogre3D and friends.
At least in order to be playable under Linux. Said this, the 99% of the games from that era will run perfectly fine with OssPD->Pipewire (install OSSPD, just run the game) and 32bit SDL1 libraries.
Quake is a rare exception. Source availability is rare on Windows, falling to almost zero for commercial applications (for obvious reasons). There's also plenty of corporate internal applications where the one company that is using it is also the only one with the source .. and they've lost it.
Quite a lot of game source is lost entirely even by the original authors.
Not to mention that even if you do have the source, changing the use of an API can be a really expensive software modification project. Even Microsoft haven't been entirely systematic, you can easily find WinForms control panel dialogs in Win11.
Some embedded Windows apps exist in this space as well. Oscilloscopes and other expensive scientific instruments that run Windows XP.
As a concrete example, the source to quake is available, this has allowed quake to run on so many platforms and windows infamous backwards compatibility has little effect in keeping quake running, windows could have broken backwards compatibility and quake would still run on it.