But every company I worked at in the past 10 years or so eventually coalesced around a singular "design system" managed by one person or a small core team. But that just goes back to my original point - every company had their own design system, and there is not a single, industry-wide set of "rails".

The bigger issue I see with "got to keep lots of designers employed" problem is the series of pointless, trend-following redesigns you'd see all the time. That said, I've seen many design departments get absolutely slaughtered at a lot of web/SaaS companies in the past 3 years. A lot of the issue designers were working on in the web and mobile for the 25 years prior are now essentially "solved problems", and so, except for the integration of AI (where I've seen nearly every company just add a chat box and that AI star icon), it looks like there is a lot less to do.