Right, without back-compat, any system trying to gain marketshare is domed.
...but Linux has chroot, symlinks, hardlinks, and everything in-between - it's entirely possible to design a Linux environment where the clutter and warts of back-compat are only visible to the users and programs that need to see them.
I’d point in the direction of Nixpkgs’s buildFHSEnv, but unfortunately Nixpkgs has its own layers and layers of cruft (being more than fifteen years old by now).
...but Linux has chroot, symlinks, hardlinks, and everything in-between - it's entirely possible to design a Linux environment where the clutter and warts of back-compat are only visible to the users and programs that need to see them.