I understand what you're saying.
Personally, I agree it's an overused expression.
Bioshock is a very fine example of how dumbed down is most appropriate - at least in my opinion.
But there's a vast difference between a game being "dumbed down/accessible" and the people playing the game being dumb or ignorant.
Some people get defensive when you call their favorite games dumbed down - and I'm not sure that's a very appropriate response.
Also, I DEFINITELY think most games were more "cerebral" in the past - meaning you had to invest more of your brain to succeed in whatever challenges you encountered.
That was just a natural consequence of catering to a less broad audience.