The principle starts much smaller than the "-isms". Pournelle's Iron Law of Bureaucracy is a classic. Another way of phrasing it is something to the effect of, without a strong external motivation preventing it from happening, the primary purpose of any organization inevitably becomes self-preservation.

You can see the effect all the down to something as small as 4 friends who have met once a month for 10 years eventually having to break up due to life getting in the way, and the feeling that not only is it going to be sad to not have these meetings any more but the feeling that there is some sort of almost-concrete entity that is somehow being hurt and needs to be defended, as if there is some obligation that has been created independent of the four participants that is being violated beyond the mere summation of four people's personal feelings. Humans build these structures readily and often defend them beyond what rationality may suggest.

Well, by definition, an unsuccessful -ism has already been shut off.

"The purpose of a system is what it does" and all that.

But yes, if a system fails to prioritize its continued existence, it doesn't matter what else it accomplishes, it will cease to exist as that system.

even priapism

Capitalism has thoroughly done its job because it's modified its hosts to evaluates itself on its own successfulness - "We investigated ourselves and found no wrongdoing"[1]-vibes.

By what measures would other systems of social organization measure their success and why aren't we choosing them?

1. https://knowyourmeme.com/memes/we-investigated-ourselves-and...