Couldn't the same argument be made for belief in God though? As billions of people on the planet believe there is a God watching over them day and night, isn't that same danger there if all those people lose that belief? Dennett has no problem at all with saying there is no God. The ideas seem similar to me as they both suggest that there is no such thing as justice. No God, no ultimate justice, that's an obvious argument. With free will it's a bit more complicated - if there's no free will, and that is scientifically 'proven' (in the minds of the general public) then that would have to lead to people not being held accountable for their actions in the way they are today (the whole concept of retributive punishment becomes a problem) - and therefore there would be no justice.
But we don't think that of belief in God, do we? We don't think for a second that people who lose their faith in God will suddenly decide it's perfectly fine to go and steal, rape and kill because there's no ultimate justice waiting for them - any more than we believe atheists, agnostics, humanists or any non-believers somehow lack any morality. So why would we think it for losing free will?
Personally I'm undecided on free will, I'm most persuaded by the idea that even though it's not to be found in fundamental laws of physics, that it is an emergent property and no less real than anything else.