I agree there's a naturalistic fallacy. Moral values aren't obviously based on statements just of objective fact. I argue against Sam Harris's utilitarianism on these grounds, for example.
Kant's moral argument for God was transcendental rather than explicitly pragmatic, but the points are similar. We need God or some other foundation if we want to be moral. That's roughly what he said.
The problem is there's no putting the genie back in the bottle. Arguably, God is dead. You can't force yourself to accept ludicrous principles just because you think doing so would be useful, not unless you give yourself a lobotomy or live with egregious existential inauthenticity. This is also where Pascal's Wager goes wrong.
So people in a godless culture would need to put their faith in something else, in something more befitting the hypermodern sensibilities. Humanists say we should trust in ourselves and indirectly in science, technology, and free society. This amounts to what I've called a Promethean or Luciferian faith in the nobility of our struggle against godless nature's absurdity and horror.