I think there might be a good measure of the number of people who are into knives - blade show attendance. If we assume that only a certain percentage of people who are into knives are going to visit Blade show, and that percentage stays more or less consistent (lets say within 5-10%), then Blade getting record attendance nearly every successive year since 2013 or so is probably a sign that there is a notable increase in the number of knife people out there.
Here's a quote about Blade 2016 from, well, Blade Magazine (same company does both, so, take this with a grain of salt as it could just be self-promotion):
"The show proper kicked off noon Friday with VIP ticket holders pouring into the show hall after waiting for hours—some as long as from the day previous—in long lines that snaked all though the show lobby and beyond. And once the patrons flooded the show hall, the race was on to get to their favorite exhibitor’s/exhibitors’ tables and booths. All told, more people attended the show the first two days of the event than for the entire show last year—and last year’s show enjoyed the highest attendance ever until this year’s rendition. A number of exhibitors commented on the large crowds, with many selling out of knives the first day."
So, maybe the industry is drawing in more consumers in a manner that we don't really see day-to-day. It's not as if 20-30% of the population suddenly got into knives, or we'd see a lot more recognizable pocket clips out there. However, even a small increase (0.1%-1%) of the world population getting into knives would make for a huge increase in knife people.
I'll bet this honestly has a lot to do with crossover interest from fidget spinners, keychain tools, watches, and guns - knives are a natural compliment and side-hobby to people who like well-made man-jewelry and firearms. Social media (instagram) makes it easy for people who are interested in one or more of those to also get exposed to - and then interested in - the others, and a lot of knife companies are making those products too. So, when someone sees a "#edc #knife #watch #tacticalpen #fidgetspinner #rolex #glock" instagram post, they might start thinking about picking up some of the classes of items they don't currently own...