Well, people need to eat, so they may have to compromise their creative vision in order to put food on the table. Life sucks like that.
Anyhow, we can look at Dragon Ball Z. Practically every arc in it could have been the last in a number of ways, and by the time we get to Cell, I'm almost certain they were trying to end the series there.
Also, you make your great work and they tell you "Oh, you've been renewed for a second season". But your best ideas already went out. Shit.