Think of it another way: Amount of people watching the show = how many are watching TV ads = how much money they will make charging for ad space
The name of the game is money. No matter how passionate the fans are, that's not what pay their employees, the actors, etc. Plus, there aren't really a massive amount of fans relative to other shows, itself at earlier periods of time, hypothetically.
The 18-49 demo is usually the most relevant of demos, but the overall demo isn't too bad either. The show has to present itself as sustainable, but more importantly profitable. The demos are decreasing and that's a bad thing, but with the new technology surge, other things come into context regarding them making money, like Hulu, etc. A show like Galavant can stick around a little while longer because ABC had no idea what to fit there since most of their new shows seem to bomb or not reach expectations.
ABC will likely keep Once for a extended period of time, until they find some show that won't backfire and will do exceedingly better than Once. Once remains #1 on it's thingy so there's no real fear. When we hear of a new hit show, or new show for Sunday nights that may be better in concept, then Once may falter. However, I'm certain that Once will last until the writers are finished with the story. If it's seven seasons as planned, then it'll most likely get its seven seasons and then ABC will do what they do.