Part of the reason why I kinda want the whole Finn knocking-up Huntress Wizard thing to happen is that it would provide a solid conclusion to the show, rather than end on ambiguity, like you mentioned.
So many epic stories like Adventure Time end on an ambiguous note, both because the writer's are lazy and because they might want to continue the story in some form in the future. There's already a good chance CN will probably reboot or continue AT in some form later, so doing this might keep them from dragging it on.
Not just that, but part of growing up is realizing that "Life goes on, but the adventure will eventually end".
It's somber, I'll admit, but I'm gonna be ticked if the show ends by just having Finn say "SCREW OUR THEME, THE ADVENTURE NEVER FUCKING ENDS BITCHES!!!"