The thing is, in a reasonably designed replacement that shouldn't matter. Requiring applications to update to the new thing just to keep working is absurd.
Using SDL is a good idea but it isn't a stable interface either, e.g. SDL 1 programs will not run with SDL 2. SDL 2 is also not something you can rely on being installed, you generally need to ship your own copy and even if you want to rely on distro packages you will need to adapt your code eventually as old versions are purged. SDL is also not a reasonable abstraction for all kinds of applications as it is focused on games and game like use-cases.
Backwards compatibility really should be the primary focus for anything looking to replace a system component. We do have Xwayland for that but its an incomplete solution by design as X clients won't see non-X windows nor can they capture the whole desktop.
Anyone with more complex cases like Wine can work with Wayland directly. Wayland protocols are pretty stable. If they care to support X too they'd need two paths.
But something like freerdp for example managed to do it with SDL well enough.
SDL of all projects actually did think about translating older ABIs to avoid breaking changes. I.e. there is SDL 1 over SDL 2 helper. It would be nice to see more of such efforts on Linux.
