This us quite a natural thing. After all.you may feel bored of one series after a point, so you want to do something different and refreshing.
In term of artists drawing fanart of various franchises, I noticed two things after a brief walk through Danbooru:
1) a lot of artists that nowadays draw stuff like Genshin, Fate Grand Order, Azur Lane, KanColle or Idolmaster used to draw Touhou in the early days of their career;
2) connected to #1, there seems to have been a big migration towards those mainstream series and flavor of the week/month gacha game; given some of the authors doing that very high quality (at least in terms of drawing techniques, not the topics depicted - maybe I should call it more professional-looking art if that's the term I'm looking for) art, I wonder what contribution would they have been to the Touhou fanart community
Anyway, is the migration a positive thing, or maybe you would have liked people to keep a tie, no matter how loose, with where they started from/come back to origin every now and then?