From what I've seen, Stanford (and maybe Harvard) are kind of in a different league here -- people actually do go teach there after a successful business career (and not even at retirement; some do it in their late 30s/40s/50s).

They also do a bit more than act as trade schools to teach skills. There's the huge networking value (only worthwhile at GSB, HSB, (+maybe MIT Sloan or UPenn Wharton for very specific disciplines too)), and they do teach some useful information beyond technical skills.

They are still probably not the best opportunity cost weighted use of time for most people (unless you want to work in the highest levels of large companies or government), but they might have some net positive value; lesser MBA programs have negative value. Getting an employer-funded MBA while living through a 2y lockup period after M&A does seem like a great choice though :)