And going to the ground in either of those cases is not far removed from suicide. At least better your chances significantly by staying on your feet.
There are many more reasons why going to the ground is generally a bad idea in a self defense situation, but if you'd rather like to ignore them, that's up to you, I guess.
Now, that's not to say that BJJ isn't useful for self-defense, or that it doesn't contain useful stand-up techniques. Just use it appropriately, and don't belittle stand-up arts for focusing on resolving a fight without going to the ground. While preparing to defend yourself on the ground should you need to is definitely a wise thing to do, it should be the last thing on your list of priorities. Why train for the worst case scenario, and the worst case position, when you can train not to get in that position to begin with? It would make sense that priorities should go something like avoidance, de-escalation, stand-up work, and lastly, ground work. If you're on the ground, a whole lot of things have already gone wrong, and you've committed yourself to one opponent and taken "running away" off of the table completely. That's just not a smart position to put yourself in if you had a choice.
Also, you must recognize that you can't just say "if, then."
"If it's just one opponent," how do you know he doesn't have buddies, or that someone else won't join in the fight, or that he is unarmed, or that you are guaranteed to win regardless and he won't get a lucky punch in or overwhelm you with brute force in a way that you did not anticipate? You won't know -- not until someone comes up and starts kicking you in the side of the head, or you're getting stabbed, or your head happens to get knocked against the concrete. Your priority should be to escape a dangerous situation, not commit yourself to it. Time is of the essence, and ending or disengaging from the fight by the quickest and least risky way possible is what you should be thinking about.