I have to disagree. Maneely was very much Stan's go-to guy. I have a hard time believing that that would've changed had he lived, which presumably means Kirby wouldn't have had a chance to demonstrate his immense creativity at Marvel.
It's obvious that Marvel, & comics in general, & for that American popular culture, benefited from the way things worked out; I just think things would've evolved completely differently if Maneely hadn't died so ridiculously young (32!).