"Then I don't have to remind you about the state of Heaven. It wasn't just a fever-dream, Daniel-- it's what's to come, in the future of my universe."

And he cannot abide an empty Heaven. Even if Gabriel could sit around in some empty universe, screwing the not-quite-locals, he couldn't.

"I shouldn't have to explain to you the ramifications of an empty Heaven while Hell overflows with demons."

Sure, he doesn't so much care about the effects it will have on humanity so much as he does the fact that his entire family will be dead, but that's just semantics. What's good for Heaven, in this case, benefits Earth as well.