I feel like this point is an almost entirely different problem. While yes, deleting old art is pretty wrong imo, and I too do not have a lot of respect for those who simply switch to waifu simulator game drawings (thanks kancolle) I think that they do not have to pay mind to what the fans want them to draw, especially if they arent doing it for money. Sometimes interest in a series can wane, and you will instead become interested in a different series.
Say, for instance, you in particular. You become big on posting topics on warhammer or something, and it gets a good amount of people to like you and follow you to see your discussion boards or videos you make on the series (i admit this point is a bit of a stretch but hear me out). Suddenly, warhammer has lost some of the luster it had before, and you see this bright shiny newer series called "touhou project". all of your attention shifts suddenly to tuhu lore and tuhuposting, making touhou videos instead of the previous warhammer ones. the fans can become confused by the change, but wouldnt you still be in your right to move to a different form of enjoyment, even if it disappointed other people? I definitly agree though it can be super annoying to find someone who suddenly stops doing touhou stuff ;-;
On the topic of them being mercenaries, i mean yeah kind of. a lot of them either genuinely enjoy the flavour of the week anime, or rely on commisions to gain income, and fortunately people enjoy seeing high quality art of the anime they enjoy. That is part of the issue with touhou being less mainstream, is that by logic it should be getting much less than we have already gotten, but fans are just so miraculously dedicated and talented that we have already gotten a lot more high quality art than other niche fandoms. some of the problems with finding high quality art feels more like an issue on the western/english speaking side of the fandom personally, as most reddit posts dont really fall into the "high quality" style you brought up, yet every other pixiv post i come across from touhou has a TON of detail put into it, its insane. if you wanna be able to see more high quality stuff i would check out pixiv directly as thats usually where the good stuff is.
Sorry if i sound kinda dumb, I'm not used to making big posts like this yet! ;-;