As much as I like zombasite and want it to be expanded upon, you have to do what makes sense for you business-wise. So if leaving the game as it is helps, with maybe a few optimizations and fixes thrown in when needed, I don't see anything wrong with moving on from it to something else. I guess it depends on how much work is involved in these expansions.
Anyway, if a new game gets here quicker because you decided not to expand Zombasite, I don't think anyone here that likes your games would fault you for it. I think the idea for your new game sounds awesome, btw. Looking forward to it, whenever it getrs here.