> every moving part would have a (small?) team of core maintainers, people that get paid well to know everything there is to know about something,

I work at a company with a few thousand employees in the Netherlands, and have been moved around a few teams in the last 7 years (not by choice, the team is nuked and I'm shuffled off elsewhere).

Technically, the product is supposed to have a "handover" to another team who can do the maintenance. What happens in practice is the person who worked on it gets saddled with maintaining it forever along with taking up the responsibilities of the newer work that comes along their way.

I've accumulated a string of such projects, including one that's used extensively by a few large companies, and the only person maintaining it is drumroll ... me, in whatever time I can spare for it.