I would say "all of the above" plus a few other things.
I agree that he sport is aging. I see a few young people getting involved, but not many. The number of shooters I see in their twenties, or even thirties, is minimal.
Not only is gas pricey, but muzzleloading is time consuming, like anything that requires practice. People are working so hard these days just to make ends meet that spare time is at a premium.
This is totally spitballing, but I also think that people are developing a different attitude towards dirt and inconvenience. Shooting black powder firearms is inconvenient, tinkery, and gets shmutz on your hands and clothing. Fewer people do hands on dirty work these days, and aren't used to that. A lot of people, especially young people, seem very familiar with computers and electronics but intimidated by craft work and mechanics.
Mostly I have experienced a welcoming attitude from muzzleloading folks, but I have run into the insider's club here and there. Luckily I had those experiences after I was well into the sport, but a newcomer might get discouraged if he didn't have the perspective of years.
One other related point, and a moderator can nail me for this if necessary. I realize that this might open a can of toxic saber toothed worms, so apologies in advance. Any shooter (like myself) with political views to the left of center had better be prepared at a shoot to self-edit his conversation and also be prepared to occasionally laugh off being described (without the speaker realizing the offense) in unflattering terms. I also ran into this in reenacting. It's not constant, or even common, but it happens often enough that I consider it. I tend to give a philosophical shrug and get on with enjoying some remote control gong ringing, but I can imagine a lot of people deciding that they were not really welcome.
I'm not saying this as a condemnation, just as an explanation - we black powder shooters are a subset of a subset of an aging demographic set (mostly) of a political demographic (almost entirely), and that means a limited group of potential participants. Again, I don't mean to condemn - it's just how things are.
That said, I think that a lot of people are just maxed out on work and kids, with no spare cash. I think that most people don't even realize that we exist, which makes it hard to recruit new participants.
I keep trying to infect people with black powder disease. I have brought out the flintlock and given people an opportunity at weekend barbecues and invited friends along to events, but it's hard to get it to catch. I'll keep trying.