One good thing about Kirby's editorship deal was that it did force him to show what he could do when forced to be a one-man band. If he'd never made that demand, fans today would never have the slightest idea as to whether he had some untapped wealth of ability overshadowed by Joe Simon and Stan Lee.
To be sure, there are a couple dozen all-Kirby scripts that I think are solidly plotted and reasonably well scripted, even taking Kirby's Kwirks into account. But many are full of plotholes because Kirby was "writing" the same breathless, helter-skelter way he did in earlier eras, but without a Simon or Lee to supervise, to urge the King to mind his p's and q's. It's because we've seen how Kirby scripted his own works that the more critical-minded fans can see how much difference it made when he had a strong editor/ supervisory collaborator.
I say "more critical minded" because I've argued with some fans who *still* don't believe Kirby didn't flawlessly write everything from the 1940s to the 1980s, and that any plotholes are brought about by interference from other parties (mainly Stan). You can show these people literal grammatical flaws in Kirby's 1970s writing and it means nothing to them, often because they grew up loving that Kirby as young readers. It's true that his quirky writing can be fun, but these fans can't see any fool's gold mixed in with the real gold.
I'm dubious as to whether an editor or scripter could've made much difference in the reception of Kirby's 1970s and 1980s work. I have a feeling that his books didn't enjoy spectacular sales because they belonged to the "gosh-wow" brand of fantasy/SF that had lost a lot of its cachet with older fans, while the 1970s marked a period when comic books were losing their hold on the younger audience thanks to bad distribution and competing media. But that's just my POV.