My guess is probably more statements 2 and 3 for the usual suspects eg Stanford
If you hire a smart kid with an MBA from stanford and he gives you a bad name with a series of mistakes, well, bad apples happen. If you hire a smart kid from a no name university and the same happens people will be quicker to blame you.
Credentialism is a thing for the same reason brand recognition is a thing. When a product with good reputation fails it's bad luck. When a product with bad reputation fails it's to be expected. Power perpetuates.
In the US, that never has been the case. Even in its golden years.
https://whorulesamerica.ucsc.edu/power/class_domination.html
Even in 1960s, 30,000 people (families, children included) dominated all US economic and political institutions. This group was an exclusive group with class consciousness, thwarting any reduction of their control and keeping outsiders out. This phenomenon continues today. Some freak successes in tech space did not change the pattern.