I can probably write an entire 10000+ word essay as to how Touhou is still going strong after 25 years even though it literally has zero official advertising.
One of the things I can rationalize is this.
You hear it all the time. "Our fans are the most important part!". "Our fans are what keep us going!".
But in most cases, while that is part of the truth, that's not exactly all of it. The fans revenue and their "free advertising" is what keeps them going, not the fans themselves. When that's gone, sooner or later what people used to clamour for will eventually wither and die. There is a lot of factors for what keeps a product/series/whatever going, the fans are only a cog in the machine.
That is not the case for Touhou. In it, ZUN is the machine and the cogs are all fans. While, at first, that may seem like a more brittle and less adaptable way of how a fandom works. After all, when the fans die, what's left?
But that is not the case for what Touhou is, or what ZUN has made it to be. In a normal case, when a cog stops, the machine breaks. In Touhou's case, when a cog stops, the machine is still functional and only gets rusty; it's only a matter of time until it gets cleaned, one cog turns and everything runs again. This is for many reasons.
One of them is because ZUN is barely affected by money, fame, or anything of the sort. He makes Touhou because he likes making it. He doesn't pander. He will keep making Touhou even if the fandom isn't as big anymore, until he gets sick of making it. This has obvious advantages, but it also connects to another important aspect.
Touhou, as a franchise, is essentially run ENTIRELY by fans, due to ZUN's stances on making derivative work.
In a normal case, if something becomes unprofitable, that something gets shut down. Even if someday, fans look at it and "remember that?" and make fanwork, it'll likely never come back, even if sometimes there's massive push for it. It takes money and risk to do such a thing, and most companies don't do such risks.
That is not the case for Touhou. Even if Touhou's popularity simmers down, all it takes is one spark, one popular fanwork, and the machine runs again. ZUN is still making games. You can still make any fanwork you like. You can still enjoy what has transpired before. Things have changed, yet nothing has. This sort of "stagnancy" is part of what makes Touhou so immortal.
As you say, dedication is a large part of it, but as to how people get dedicated, that's more complex. It's something I'd have to rant about even further.
There's a lot of reasons as to how Touhou has become what it is today. The type of content it brings, the lack of pandering, the dedication of its fans, what advantages good music brings over other types of content, etc. This is just one thing I've noticed.