I think you forgot one case : it’s psychologically extremely hard to abandon something you’ve been working on / believing in for a huge part of your life, and start over from scratch, no matter how smart you are ( i’d say the smartest you are, the hardest it is to believe you’re completely wrong) . So teachers could be tempted to hang on to the narrowest hope their work still has meaning and keep taking new phds and this could last for a very long time.