I've gathered the circle numbers information from the past 8 Comikets, and I've noticed a few things...
c85 Touhou 2272 Kuroko's Basketball 1462 kancolle 1136
c89 kancolle 2747 Touken Ranbu 1724 Touhou 1666 idolmaster 1325 | c86 kancolle 2038 touhou 1918 Kuroko's Basketball 1444
c90 Kancolle - 2678 Touhou - 1625 Touken Rabou - 1482 IdolMaster - 1328 | c87 kancolle 2552 Touhou 1756 Kuroko's Basketball 1178
c91 kancolle 2824 touhou 1623 touken ranbu 1501 idolmaster 1479 | c88 Kancolle - 2566 Touhou - 1508 IdolMaster - 1186
c92 kancolle 2329 touhou 1320 Idolmaster 1299 Touken Ranbu 1244 Fate series 1129 |
(All series with >1000 circles at Comiket)1. Touhou is not the darling of the doujin scene anymore, Kancolle is.
2. It used to be the undisputed queen of Comiket. Now it's fighting for second against two or three series. (Ceding it briefly to Touken Ranbu at Comiket 89.)
3. While everyone's representation was down (except for the Fate series), Touhou's Comiket 92 representation is the smallest in years.
IMHO, Touhou's time at the top is probably over. As to why? One, Kancolle is "new and exciting", while Touhou is "old and stale". Two, Touken Ranbu is a big hit to a fanbase Touhou never really played to, the fujoshi, so it's here to stay too. Three, everybody that has its roots or a presence in mobile gaming are increasing, which is extremely popular in Japan, while Touhou, whose games are PC based, a platform that's niche at best in Japan, is shrinking.
As for my first point, I don't think it's too difficult to see Touhou as a series as old and stale. As a series it is ever expanding, but not evolving. Negative Continuity,
is a problem for the series. Especially after the MoF-TD chain showed the series can work with storyline continuity, only for ZUN to backflop back into the old way of having each game having nothing to do with each other. You can retcon completely any game from DDC to HSDS and it would not affect the series universe at all (save for losing a few characters, most of whom wouldn't be missed).
And the characters aren't much better either. For the most part, most of the series' characters remain unchanged since their debut (if they are ever seen again).
Gameplay mechanics wise. Outside of a one-time gimmick, they haven't really changed at all either. Same linear progression (except IM, which had two different Stage 6's, and PoFV which was practically a different genre altogether), same core gameplay systems... It's a stale series.