1. That's my bill for hosting and bandwidth expenses. I obviously need to consider the cost of my time which is not included in that figure. The more money the site generates, the more time I invest in it. So, if it were to generate $150,000 from advertising, I wouldn't shut it off, but I'd start pulling my time out of other projects, and moving it into this community site to add more of the requested features.
2. I want to save for a rainy day. Online communities don't last forever. It would be short sighted to shut off advertising simply because this years expenses are met, and then two years from now I need to close up the site because advertising is falling short. When the site hits a rough patch, I want the savings to either push it through or pivot. Coasting on maintenance costs would be a dangerous road with an abrupt end.