I dunno, i think its a bit short-sighted to say that he didnt have martial arts as we have it today - Kung Fu has been around for a few thousand years, many other martial arts have been around for times ranging from 20 to 500+ years. How long has MMA been popularised in the west in the form similar to what it takes now?
A few decades at most? (Im half rhetorical, half actually asking here

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Bruce Lee hasnt been dead THAT long. Over a millenia of cumulative knowledge on martial arts by the Chinese people that was passed on to him isnt going to be beat out in the relatively short time since he was in his prime.
Its common sense to say that he'd have a tougher time with heavier opponants because thats just how it works, but theres no substitute for innate ability. All the training money can buy cant compete with "a natural" and thats what BL was. I dont think he was ever actually beaten, or even came CLOSE to being beaten.
Few fights are a walkover, and sure its a different stage now compared to back then, but to say that in his prime he has no chance against ANY modern MMA fighters is like saying Ali in his prime would have no chance against ANY modern boxers.
I know its not an exact comparison, so dont flame me for that, the point is that both those guys were always gonna be the best at what they did no matter when they were born.