Who knows what forces around their capabilities that have evil intents, the Q are keeping at bay. Who knows what power vacuum will be filled when they are gone.
And for every malevoilent Q, there's probably at least one out there with good intent doing good things.
Actually, it's only the 'rogue' Q we've seen who are actually doing anything at all. Remember Death Wish? Most of the Q are so bored with immortality that
they literally don't do anything.
So if the entire Q continuum managed to disappear, there would be no power vacuum left in their wake - only the lack of damage being done by rogues like 'our' Q. The Q, in general, aren't keeping the universe safe - they're just sitting around and doing nothing. Because from their perspective,
it's all been done.