This really happened about fifteen to twenty years ago. I temember a BJJ blue belt coming to my Training Hall in Alma and getting submitted repeatedly by everyone training that night. Once it is a business the standards fall. Look at all of the celebrities who have belts from private only lessons and have never rolled with the masses. Most BJJ blackbelts of today are not even close to the fundamental skill sets of purple belts in the nineties. The one thing they do have in general though is some very excellent new submissions and leg lock skills that BJJ pioneers have advanced!
I would add though that at the competitive level in BJJ the practitioners are at a higher level of skill set. It is just the recreational level or the hobbyist that the skill sets per belt is lower than it used to be!