My idea: Touhou is a Supernova.
Its width of products will expand and expand and soon enough beyond it be damned by its own popularity. Just like a Supernova, it won't be soon before long it will be silenced to nothing, fed upon by its own popularity until nothing is left and boom, oh hey, no one draws Touhou anymore~
It is not that I don't like Touhou doujins, heck, I have more of them than I do of miscellaneous others, even the strange fetish ones that probably no one likes to read other than the niche few (Breast enlargement anyone? NO? I figured so). The doujin market is described as best like any other bubble market, it grows susbstantially because of the supply and demand curve marking a heightened sales level and status, giving it its 'premiere' level of widespread influence, making anyone in the market of anime and manga move over and check it out. In the event they find something they like and they see that they are not the only ones, the buy becomes buy more.
The same with the building boom in Dubai that crashed recently not so long ago. Investments (think of this as the doujin producers) were into the building of so many new infrastructures, facilities, business buildings and expensive villas that nobody thought twice of how good it will go bad at the end (the higher they go, harder they fall). Doujin without general interest spawns nothing, but because of the fanbase it expanded exponentially.
In due time, when markets go bad for Dubai, investors are indebted to banks, and now relies on Abu Dhabi to bail them out. Think of when the doujin market will suffer the bubble burst, and when it does, the interest will just die, and those that came late to the doujin drawan market and cannot divert their artistic license to anywhere else, it be sad to see them go, but yes, they will be dead too. Only loyal fans may live on a franchise, but those that contribute to the bubble boom will die along with the bubble burst.
/economic analysis