Well, it means they have to keep track of every build, and keep every version of every OS live, and that's not something they really do. Especially in cases of security fixes, they'll take down an old version from the update servers.
As for 15min, they're actually testing fast patching right now, although it's currently limited to security updates (rapid response).
That's not a lot of work. I would be extremely surprised if they were not already doing it. If someone reports an issue going back to a specific version, they need to be able to install that specific one.
I think you're overestimating the effort / underestimating Apple resources here. Ubuntu keeps all released packages, going back to 2004 online for example. (And isos, even for the effectively dead powerpc) Microsoft's symbol server is likely also a way bigger project than just preserving images of all versions.
> and that's not something they really do
Are you actually speaking for Apple engineering here? The version not being advertised on the update servers does not mean it's not available.
Also, there's the question of supply chain audit. I doubt their security would accept not being able to say "this is/isn't what we shipped at the time".
This feels weird. So they have a very low number of possible systems to support. The OS is immutable. Why would they go with a reinstall at that point instead of "we know which part failed, so we'll re-download that one block from Apple service"?
(I guess that one ties in with: so you've got a low number of systems and an immutable OS - why does it take 15min to apply an update)