This is heavy speculation (as was the above paragraph :V), but I'm also wondering if there's some growing resentment towards Touhou overall from doujin fans who aren't into Touhou. Going by what someone said about Touhou making up about half of all doujin (probably a huge overstatement), the present situation has to suck for non-Touhou doujin fans. I know that if I were in such a position I'd probably start reconsidering my hobbies and stop making regular trips to Comiket and doujin shops if all I saw was all Touhou, all the time. Applying this to the general doujin fanbase and I'm wondering if traffic to Comiket and doujin stores will decline, and we'll see second party (i.e., vendors) regulation of Touhou supply, perhaps opening the window for something else to take its place.
The doujin craze of the comicket is partly there for people who want to get in to the field of manga, games, music, anime... etc.
Its just that there are also lots of people who come to enjoy that as a sort of party and lots of people who come to target that certain market.
For example, many of the manga artists that we see today has had experience making doujins in the Comick market.
A famous example is the artist Koge donbo.
So, even if the majority of the stuff is Touhou, it wouldn't matter for people that are actually there for the Comick market.
People who go there to get "entertained" or feeling like they're "guests" or "customers" or something.... THOSE are the people who shall get lost quickly.
(Or at least that's the ideal of these events. Same goes for the Hakurei Shrine Reitaisai)
Another thing to note is that doujin artist rarely do stuff like "only on series", and that's because they will put "what they want to make" in front of what'll sell.
(One of the doujin artists that I follow went from Leaf and Kea only all the way to making Gundam, Geass, and Touhou stuff. Mind you, I actually do go around to their home pages and check out what thay're up to. Been doing it for quite some while actually.)
So its not rare to find something like a Magical girl R18+ doujin beside a clean Touhou doujin. ... Or even a JOJO doujin artbook.
The only places where your speculations can take place are doujin shops, but the guys running those stores are quite smart too and they do a good job of balancing out what they have as well as checking out the market too.
When the fanbase will fall is most likely when ZUN decides to quit Touhou, and new stuff stops coming out. (Or ZUN dies from alcohol overdose)
-Which is unlikely to happen since he said that he makes the games for his reliever from actual work.-
or when it finishes going through its prime. ...Although we have yet to decide when was the prime... or has the prime yet to come?