> there's only a small handful of martial arts that actually work in applied person-to-person combat
Even MMA, which is billed as a sorting of martial art's effectiveness, has substantial rules that greatly limit the fighters. For instance, no groin strikes, leg stomps or eye gouges: all of which are actions I've heard are the first thing you should do in a real fight!
Not arguing they should be legal (sport is brutal enough as it is), but even MMA, while closer to "real combat", has severe limitations and arbitrary features.
> all largely useless in the real world when faced with multiple opponents
^ This, 1000x. If you get into a fight you were not planning, it is called an ambush. The most logical action is to run the hell away, or at least, use absolutely overwhelming firepower and dirty, dirty attacks. The concept of two folks trading blows is not how real fights actual progress, let alone two kung-fu masters jumping between buildings...