Personally, I think no incentives are needed...
I know from experience that if there's no time or desire, nothing will make you play, and most importantly, you shouldn't force anyone to do anything – otherwise, it could lead to addiction...
It's great when the desire to play the game coincides with the next tournament...
Regarding the current situation with offline play, don't forget that online play is a NEW thing for us (HW)!!! and it has simply temporarily replaced offline play...
Not everyone who played offline in 2007 has left the forum – they have simply switched to online play en masse... but will it last long?
Regarding the statistics (my opinion):
HMM1 and HMM2 are classics and they consistently have a "reserve" of players... that is, even in bad times, there are potential participants who decide to revisit the past and immerse themselves in a world as close as possible to natural chess.
HMM3 is the most playable installment, and although the offline statistics are poor, people haven't left... they're all in a different discipline (online).
HMM4 – no comments.
HMM5 has both online and offline play at a decent level (and what else could you expect from the latest installment in the Heroes series, especially with the recently released expansion).