I'm constantly bedeviled by the 'core game' question. Our GW sales companies love labels. What they mean by core game I can only dimly imagine. What I mean by 'core games' is specifically Warhammer and Warhammer 40K which, taken together with their product ranges account for over 70% of all our total sales and will probably continue to do so for the forseeable future.
So whether a game is 'core' or not has nothing to do with its longevity or support - at least according to my definition. In the case of Warmaster I've deliberately set the game up so that it can be produced in relatively small numbers (book only, no expensive card reprints, no packing, little warehousing). The same goes for the model range (all blisters which are easy to make rather than boxes which are expensive to make in small quantities).
And I've started off modestly with a range that can grow organically (no plastic models, relatively little design time). The sum total of all this is that we have a game that is much more likely to remain part of our ranges even if the uptake is modest - it's engineered to survive!
RickP