I never really believed in the Progress theory of History (things are always improving as history progresses). Some things are better now than previous eras and some things are much worse. Not everyone believed that women and non-white people were inferior any more than everyone believed the earth was flat. The thing that got everyone's (well, Michael's) attention was that no one got into the Good Place for 500 years, coincidentally about the time the modern world was beginning.
As for Brent, it kind of reminds me of what Machiavelli said: "I desire to go to Hell and not to Heaven. In the former I shall enjoy the company of popes, kings and princes, while in the latter are only beggars, monks and apostles."
Studies have shown that wealth deadens empathy, so I can see how Brent could be the way he is, not really having to care about the "little people" in the 18 years he was growing Norwalk Materials from a $90 million to a $94 million business.
For people in the elite, that becomes part of their identity, (Tahani was in a similar situation until she actually did some serious self-examination). Now that in the "Good Place" there is no elite, that part of Brent's identity is being challenged (whether he is oblivious or in denial we don't know yet) like he never had been in life and if he never bothered to expand who he is beyond "Elite Princeton Grad, head of Norwalk Materials" his whole identity could collapse. Or as Bad Janet said: Boom! They turn into babies.