When will people understand that any centralized auth-related service is always going to be a prime target for criminal forces (hacking or not)?
One really doesn't need an IQ higher than 50 to understand this.
There are a few security mantras that I wish we could get through to people on, but we security people are often guilty of having a correct answer that moves around based on what just occurred.
Usually that phrase refers to libraries made by experts, not services that might be setup just as amateurish as one's own, with the drawback of monoculture on top.
However, that is only one factor among several that are considered when making that sort of decision.
I don't think there's an obvious answer or ideal, but I've also had trouble finding more comprehensive discussion on the subject.
Separate websites and online services having their own authentication bubble but implemented with industry standard libraries would probably be a better alternative to both.
People's IQ is not constant, especially in management. It depends highly on money and the amount of buzzwords that hit their ears.