If we were to raise Bruce Lee from the grave and put him in the prime of his life, would he win? No, I don’t think he would, but that’s not fair, seeing as Bruce Lee was raised in an era where Jiu Jitsu didn’t really exist, and collegiate wrestling was not considered a major part of martial arts. However, let’s say he was born in 1990, and was raised in this era of MMA, I think, by the time 2020 hit, he’d be a legitimate contender. He's got a peerless work ethic, fantastic conditioning, and an open mind to picking up new techniques.
The skills of somebody from half a century ago would be out of date in today's world of mixed martial arts, but MMA wouldn't be what it is today without people like Bruce Lee as a part of it's history. Bruce Lee is an icon to many top mixed martial artists today, including 8 year undefeated middleweight champion Anderson Silva.
Dana White was once quoted on the subject and said, “Actually, the father of mixed martial arts, if you will, was Bruce Lee. If you look at the way Bruce Lee trained, the way he fought, and many of the things he wrote, he said the perfect style was no style. You take a little something from everything. You take the good things from every different discipline, use what works, and you throw the rest away.”
Just as it is perhaps naive to think he can battle against everyone decades after his passing, it is disrespectful to go "he'll lose because he was born 70 years ago". Well, yeah, just don't be a jerk about it. I think he would have realized rather young, which sports worked in the UFC, and which ones didn’t, and would devote his life to the martial craft, and by the time he reached fighting age, would be a strong competitor.