Postdocs used to be an exception, something prestigious you did before settling into a permanent role that might be a lower-ranked university. If you were really good, you might do 2 years at IAS or MIT before taking the position at a place like UCLA or Michigan (which are still excellent, just not as highly ranked). Then it became something you had to do, before getting any job--mandatory low-wage labor, another rung on the ladder. Now, it seems to be going out of fashion, as the prize loses its appeal.
I'm surprised this didn't happen 10 years ago. The academic job market has been a toilet for 30+ years--and this change is a good thing, by the way--and I had expected bimodalization to come much quicker.
You're absolutely right on the "labor shortage" bullshit. There isn't one. The working class has been fully depleted and there isn't any more give. People are shutting down. Workers who can't afford gas are forced to live off other people (either through illegitimate labor or by dependency.) This is the turtle at the bottom burping. About fucking time.