If the application is built against 2.12 it may link against symbols which are versioned 2.12 and may not work against 2.11 - the opposite (building against 2.11 and running on 2.12) will work
>If true (I don't think it is), that is a hard showstopper for most companies that want to develop for Linux.
Not really a show stopper, vendors just do what vendors do and bundle all their dependencies in. Similar to windows when you use anything outside of the win32 API.
The only problem with this approach is that glibc cannot have multiple versions running at once. We have “fixed” this with process namespaces and hence containers/flatpak where you can bundle everything including your own glibc.
Naturally the downside is that each app bundles their own libraries.