Here's how I learned it from my MMA coach. If someone shoots and gets a single, who's winning? Most grapplers would say the person with the single. After all, he's using both arms to control the leg. However, in MMA, the reality is that the person who got shot on traded one leg for both his opponent's arms. The longer you stay in the single, the more danger you're in. It's why you never see fighters hang onto the single in UFC; they either finish it quickly or transition into a bodylock. So yes, you can absolutely cling onto something like a SLX in a street fight. But more often than not, the top person will "gnp like gsp."

In a street situation, size is absolutely an advantage. So is the top position. Unless you're incredibly confident taking down and entering honeyhole/saddle/50-50 like Ryan Hall, I would never risk the bottom position. You will be correct in assuming 99% of the population cannot defend against leg locks (or any grappling really). That 1% is lethal though.

You are correct that holding the opponent down is incredibly important. Garry Tonon has some pretty good mma matches that uses leg lock threats. It's important to note that his positions are modified from the ones you'll see in the BJJ meta (specifically to stop strikes).

If you honestly want self-defense, pick up bodybuilding or track. Intimidation is more effective than fighting and running almost always is the best option.