Of course, the problem with that is if the ratings really suck, you're suddenly in a serious race against time trying to salvage something before the whole enterprise implodes because no one's willing to watch it anymore.
And every time that happens, it makes studios a little less likely to be interested in trying again in the future.
The problem is, the "Core" fans are too small an audience, so, if you're doing so poorly that you need to down to them, you have already lost, it's already too late.
And that's assuming you have 100% of their support. Say you're down to the core fans, what do you do when you have only 50% or so that already small number backing you?