[LeFantome]

It depends. First, I think the kernel point is a powerful one.

Second, until Wayland, X11 also created crazy backwards compatibility. You can take run X apps that are decades old.

Where backwards compatibility is poor is in the desktop libraries like those in the GNOME and KDE ecosystems.

Even there though, the old libraries still work. You can keep them around if you want.

[freedomben]

Indeed, and with tools like Distrobox nowadays, it really is realistic to have a Fedora 40 host system running apps compiled for Fedora 17. In related news, being stuck with RHEL on the host has gotten a lot less painful. Desktops are finally really benefitting from containers (including flatpak)!