If you're only talking in hypotheticals then they're already optional and replaceable. Someone just needs to do the work to replace them.

You're missing that these parts have to get replaced for a practical reason. Not just because someone vaguely feels it would be more fault tolerant. For example, think of some other libc that's optimized for a certain hardware. Distros that don't support that hardware would have no reason to ever use that libc. If you say all distros should support it, then your opinion is really "all distros should support this random hardware that might be rare, expensive, highly specialized, hard to develop for, etc" and now that's a much more complex and demanding task you're asking someone to do, for very little benefit.