Im sorry but I have forgotten more about Martial Arts and especially crossed trained MMA than you will ever know. During the era you claim, yes, there was no weight classes. However, people now are so well crossed trained that even BJJ black belts have a hard time submitting people who know submission defense. Look at the Title Holders in just the UFC. Only one (BJ Penn) is a BJJ guy first. The rest learned out of cross training and came from a karate or wrestling background.
If you still want to go back in the day, I can show you COUNTLESS mistakes by the bigger guys against Gracie that fighters just dont make. Example from just UFC 100. Lesnar didnt posture up so there was no way Mir could attempt a submission. Watch any of the old UFC fights and people leave out arms, necks, drop arms in the guard, got easily swept, and walked into chokes.
BJ Penn (who is one of the best BJJ guys in the UFC) lost 2 of his 3 UFC fights to Welterweights which is on average 15 to 20lbs heavier.