"For a martial artists like Bruce Lee back in the old days who want to win and survive in life and death battles with such opponents, which weaknesses can you exploit besides the crotch to have a chance to beat him?"
Joints, which include knees, shoulders, fingers, wrists, elbows, ankles, and the hip joint. Eyes. Ear drum. Temples. Upper cervical. Spine. Kidneys. Liver. Spleen. Heart. Solar plexus. A bunch of nerves with a bunch of names in a bunch of locations not covered by heavy muscle. Primary arteries. Primary veins. Those are restricted to constriction or massive impact trauma in H2H, although with a knife edge you could cut them using far less work.
"For obvious reasons, trying to compete with him on the ground, or slugging it out with him would be suicide since when he has you on the ground, odds are you're pretty much going home in a body bag"
True, fighting strength vs strength when he has the advantage, confers you no advantage.
The human body can take an amazing amount of non-specific trauma such as bruising or what not. The human body, however, cannot withstand specific critical damage. It's the difference between shattering the window of a car and planting a bomb in the engine and blowing the engine sky high. The car still works in the first instance. Not so in the second. You can beat the superstructure or body of a car a lot and damage it and make it look horrible. But if you don't destroy the actual engine, controls, wheels, or brakes, it'll still work.
Those targets are the ones off the top of my head. They aren't anything special. Any 2nd year medical student should know them generally and sometimes specifically. The key to H2H fighting is knowing how to access those targets with an application. Theory is no good if you cannot bring it into the real world.
Thus most people, doesn't matter what their education or intelligence level is, knows that the eyes and knees are weak. What they differ in is how many attacks they know that can effectively access the target.
A light scratch across the eyes to make it water is very different from a deep penetrating impact with the finger or knuckles to shatter the globe of jelly and make it leak out cause you stirred it up inside and scooped it out. The two are completely different attacks with different intentions. Yet it's the same target.
Btw, people treat guns like a cult idol. It looks all powerful but they forget that all ranged weapons lose their effectiveness at two points. Beyond their range and when something gets too close to them. It's the same for spears vs swords. It's the same for pikes vs longbows. A ranged weapon is only good when the enemy is within range. Not when it is out of range or too close past your range. The secondary attribute of ranged weapons is ROF: rate of fire. The slower the rate of the fire, the less effective it is at keeping people at a certain optimum range. Obviously, a jam or a need to reload, reduces the ROF to zero for a time. Same for people who can't handle recoil because their stance is incorrect. It reduces ROF. It also reduces accuracy, which is another issue.
Thus hand guns only beat hand attacks when the gun is able to shoot at the H2H user out of H2H range and when the gun is able to have a ROF and accuracy high enough to take out the attacker before the attacker engages the gun user in H2H range.
The line that says "don't bring a knife to a gun fight" typifies certain popular misconceptions about the power of guns.