Reagan fired a bunch, and then (naturally) hired a bunch to replace them. ATC work, generally speaking, for twenty years (that's when their pension vests), so twenty years after the strike there was a "cliff", with a larger than usual number of ATC retirements. As I understand it, that was anticipated at the turn of the millennium, and hiring + training ramped up to compensate, without much disruption. The next "cliff", twenty years after that (ie, that millennium tranche retiring), coincided with 1) a less than forward-looking administration, and 2) COVID. We still haven't dug our way out from under the second wave of retirements.
You're absolutely right that solutions should have been taken, but it's also true that we're picking up the pieces of a decision taken forty years ago.
Source: /r/ATC. I highly recommend lurking there.