I know several research groups where the PI's sole role is fundraising, despite them having much more funding than the average research group. My point was more generic--it's not obvious to me why you would expect groups to think "okay, we have enough resources, let's stop trying to acquire more" instead of "okay, we have enough resources to take our ambitions to the next stage." The American Cancer Society has about a billion dollar budget, and yet they aren't saying "yeah, this is enough to deal with cancer, we don't need your money." (It may be the case that a particular professor stops writing grant applications, because they're limited by attention they can give to their graduate students. But it's not like any of those professors will say "yeah, my field is big enough, we don't need any more professor slots for my students to take.")