He was definitely still serious about it. But from reading the bio it's clear that he was at least as focused, if not more so, on other things. It also seemed to me as I read it that the whole experience of Mexico City left such a bad taste that he wasn't completely sold on going through something like that again, though he wasn't completely sure that he didn't want to.
The thing is, when you're mentally past it you aren't going to run as well as when you were totally committed. Going back to Elliott, in one of Brian Lenton's books Lenton comments about "never seeing the best" of Elliott, referring to Elliott's early retirement. Elliott replies that people DID see the best of him; that once he'd won the gold and was married he just didn't have the desire to do what he'd have to do to do better than he'd done.
He talked about starting a comeback for the 1962 Commonwealth Games planning to race against Snell and then scrapping the idea because he recognized that he just didn't have the desire.
When people talk about Ryun fulfilling his potetnial the assumption is usually that he had more room for physicsl improvement. But if we use Elliott's criteria, we probably saw the best of Ryun.
Ok, so what would have happened if circumstances then are like they are now? He'd have had a fat shoe contract, probably done some commercials, given his level of celebrity, made loads of money in prize and appearance fees, and not had to worry about supporting his family. So maybe he stays in the sport for another decade and is the first under 3:50 with a gold at Munich. But who knows, maybe he overraces because he wants the money and is never as good as he was in 1967.