Edit: I found the psychological effect I talked about the dunning-kruger effect and here is an interesting write-up on how that comes into account for multiplayer games but you can easily see how this is applicable to any competitive activity in general.

Most people, in any sport/activity, will not get anywhere near a black belt level no matter how long they train for. Regardless of whether the sport is exploding or not, I would guess the average person could get to purple belt - eventually. Past that requires huge numbers of hours in training/drilling/technique work that frankly most people are not passionate enough or don't have the means to go past. Take tennis for example, the basic breakdown of divisions is 2.5, 3.0 (what I consider encompasses white belt to BJJ) 3.5, 4.0 (blue belt), 4.5 (purple) 5.0-5.5 (brown) and professional (black). Most guys will play for 30+ years but never get past low 4.5 - they aren't interested in taking lessons and drilling their forehand technique over and over and over for slightly better results progressively. You'll also see guys that will come in, get really into it, taking weekly lessons etc that in a couple years will surpass almost anyone around them that isn't a college-bound junior player or a previous college player. The same is true for just about any activity that requires some balance of athletism, technique, and mental dexterity. For better or worse, BJJ and tennis both have a huge mental/technique aspect. It's awesome as you don't have to be the biggest/strongest to still succeed, but it also can frustrate a lot of 'athletes' that can go into other sports and do well by getting enough technique to survive and then bullrushing their way through those smaller guys.

I can't remember what the actual name of the hypothesis is - but this famous psychologist put forth a proposal called the something-or-other effect, that basically said lower-level people are actually so bad at that activity in comparison to higher level people that they literally cannot process the difficulty in progressing upwards. They see their huge linear gains in the beginning and expect that to carry through, when in reality, many of the nuances that separate the top echelon people are things that take hundreds of hours of practice to obtain a 1% 'increase' in that aspect. This in turn makes a lot of people believe, well if I started earlier, or had more time I could get to that level, look how well I'm doing now and how much better I'm getting. In reality, most people experience those initial gains, but its the people that are willing to absolutely stick through it for countless hours for marginal additional gain are the ones that slowly separate themselves.

Tl:dr - I highly doubt you're suddenly going to see this huge influx of black belts