I think it was actually higher! Like, at least some 50%?
Something you said I agree with, Elmy: We just didn't have that many options! I played Bart vs The Space Mutants so much, even though I knew (even back then), that it was not that great a game...
Another thing about the olden days is: Costs were lower, content was less complex. Nintendo limited publishers to 5 games a year, and even then, look at all the carp (fishy reference intended) that came out! Standards and costs for content have risen...
Publishers can't afford to pump simplistic games out the door. I think back then you could feel a bit more "passion" coming from dev teams, whereas nowadays it's all just a giant business machine. I've sinned, right there...