No doubt there’s “wear and tear” after 50 years, and even concrete is not indestructible. But I can’t get out of my mind the sidewall drains that weren’t operating in the area of the failure. That water was going somewhere. Underslab erosion? Slab jacking? No matter, it was an upset condition, and should have been addressed.
The operators knew it, yet took no action. If us novices can see the problem, then those “professionals” who we taxpayers pay to perform should have known.
My guess is underslab erosion. We can see now that the missing section of spillway was built on what I’d call lessor rock. All other issues appear to be secondary to that. In my opinion, of course.