Personally, I have been looking at this thing from another point of view. Think of an artist (or any sort of creator) that builds up a fanbase around them. The people gather as they like the art of the person, or they like the topic they are drawing. After a long time of familiarizing your fanbase with one thing, you are all of a sudden switching to a completely different franchise, without any warning or explanation. In some of the cases, the artist may even delete their works related to the previous franchise. Those who have been around since the artist used to draw what they used to enjoy are now confused, not really understanding why the change (bonus points if they never address again the previous franchise they used to draw from). While the new topic may gather new fans, you have the chance of also losing the ones that really appreciated your art for what it used to depict.
Some of the details in here are very specific because way too many times I have met the situation, even in our fandom. Artist draws cool Touhou stuff, then suddenly switched to another series (bonus points if it's a gacha game series/any other series with easy to lewd waifus). Yo find their old Touhou art on an image platform like Danbooru or Zerochan, want to see the original upload on Pixiv/DeviantArt/Twitter/whatever other platform, but you see a "this post has been deleted". You go back to the image board, see the artist has more Touhou stuff. This time, you just want to check their Pixiv/DeviantArt/Twitter/whatever account as a whole. And to your surprise, there is no more Touhou pictures there - everything was deleted. Personally, I believe deleting your old art is a bit of a sin. Not only do you leave a false impression be not showing any great artist started from low, but you also erase a part of your artist identity and past. Just like when trying to erase parts of history just because they do not fir your agenda anymore.
To me, they (this kind of artists) are basically just mercenaries nowadays, switching from one series to another together with the trend. Latest anime series, this flavor of the month game, you name it. Whenever something falls out of fashion, the artist will also switch to the next big thing. Also why call them "mercenaries"? In the past, mercenaries used to be those professional soldiers who used to fight for the one who could pay them the highest. Nowadays, I would call artists as such since they are also can get paid a lot by milking popular franchises of the week/month, as well as these series giving them basically infinite resources to lewd - let's be honest, a lot of the designs from the series listed below in the spoiler area basically have a big sign next to them saying "pervert me" - totally on purpose because of the original game design for that characters - that, and people being horny after ship girls, gender-swap historical figures and CCP Chinese gacha game (all hail Zhong Xina!).
Overall, I just feel the idea Touhou fanart is rather low(er) in quality than what those mercenaries are producing nowadays for other franchises and their fandoms. But I think the element of Touhou fanart that has a special spot in my hear is the great variety of people it brings together, everyone with their own style. "Life is like a box of chocolates. You never know what you're gonna get." is a rather pertinent idea for presenting our fandom, so unique in its way of being and creating their content.