You're really underselling Durant's smarts. Look at his numbers over his career, his TD:INT ratio has gotten better each year he's been in the league, especially during the last few years. His decision-making has really improved. He's clearly not just getting by on his physical tools.
I think it's really tough to forecast player development. Willy does a ton of great stuff already, he escapes pressure, he finds the open man, but he also throws a ton of picks and he's got this weird obsession with questionable corner-route throws in the red zone. Is he ever going to clean up those parts of his game? Who can say?
I don't think I'd trade Willy for Durant straight-up either, because I'm an optimist and I think that Willy will clean up the messy parts of his game, and that we'll get more good years out of Willy than we would out of Durant. But I think Durant is a great QB in this league and should be respected as such.