What the **** is an "average street thug?" What does "good at said MA" mean? I got jumped at a party and I reacted with movement that can be seen in an aikido class. I wasn't a black belt then. What qualifies as "hard stuff?" Anything martial technique can be done 'hard' or 'soft.' If you are indicating that some martial arts are generally more dynamic with their contact...well I can understand that. But also keep in mind that the more popular a martial art the more shitty ass dojo will pop up and provide garbage "MMA" or "MT" lessons to make a quick buck (going back to martial arts popularism). My point being you can't really judge a person solely on what they train in but how well (hard) they train, and what their training mentality is.
Oh, and a BJJ black belt can't really be compared to any Japanese martial art black belt. Its a totally different system with a different mindset as it relates to promoting. I have no authority in which to make this next comparison, but I kind of liken a BJJ black belt to a 4th dan in a traditional Japanese art.
I don't think it's a question of it being popular, rather than why people do it, without any regret. I have been doing Aikido for 3 years now (previous background in Kendo and Kick Boxing) and I love it more day by day. I'm within the Aikikai style, but I follow a certain Shihan (I'm not going to say who) whos approach is dynamic, interactive and uke is attacking 100%. It's hard, I get a "rough landing" in reality sometimes but I love it. I know it doesn't have all the answers to all the questions, but anyone who says it's a complete art and doesn't require more research is wrong (for me). Don't be hesitant about what you are doing, but find a good reason to encourage you to keep going, without the slightest of a doubt, that's my opinion.
The notion that something gaining or losing popularity, based on quantifiable results, assumes universally-logical decision-making on the part of humans.