Watch some old tape. It puts things in perspective.
BJ's boxing was elite -- against guys that didn't know what a jab was.
BJ's BJJ was elite -- during a time in which fighters still got armbarred in the guard, pounded out from mount, and the Gracies were thought of as the world's best grapplers.
The games evolved in both areas. Striking is at an obviously different level today. Grappling has evolved to a different level too.
BJ's static jab-cross with no angles or footwork boxing and no kicking would suck today and his outdated grappling skillset would just be pretty good.
The game's evolved. Khabib and Conor destroy him.