I've never understood this argument.
Is the average NBA, NFL, MLB, etc better today than 20 years ago? Of course. No one would argue otherwise.
Is the average MMA competitior better than 20 years ago? Of course.
Training methods, access to knowledge, etc- all improve over time. Why wouldn't bjj be any different?
The average competition black belt today is light years better than the average competition blackbelt from 20 years ago. Yes, perhaps the best from then would wreck the 45 year old hobbyists today, but that is not an apples to apples comparison as there wasn't that type of blackbelt 20 years ago (in america at least).