>There's nothing "rare" and "very specific" about parallel processing, what's "rare, very specific" is the amount of software that's been rewritten/redesigned to take advantage of it so far.
People have been banging their heads against the 'rewrite this software to take advantage of multiple cores' wall for decades. The lack of progress is telling. For a straightforward example, look at the second half of this Factorio update blog: https://www.factorio.com/blog/post/fff-215
Factorio is a sim game that you would think on first consideration would hugely benefit from a multithreaded design. It turns out that doing so is actually slower(!). And although this example is pulled from a game, it is essentially the same story again and again, no matter what the subject.
Now consider the second half of your statement- that the main benefit of multi-core processing is that it provides more CPUs, so that if any one gets choked, the general environment continues to operate.
(Which is true, and a great advantage of having a multi-core CPU.)
But consider a little deeper, too. If the first, best defense we have regarding multi-core designs is that they are simply more single-cores to have on hand, what does that say about the relative value of parallel processing vs. single-thread performance? Inherently serial workloads dominate across the board, in every field. The few parallel problems we have, we have because people have put a lot of brain sweat in to figuring out what, exactly, we can even do with all these cores lying around.
Meanwhile, there are entire classes of problems that are simply waiting for better single-thread performance before we can move ahead.
This is a very real problem, and it isn't going away.