Lebron on the downside is a stretch. I mean he's still easily the best player in the game and will remain that for probably another two seasons if not three.
Durant can't do the things that Lebron can do.
Also Tim Duncan another former #1 pick was in his second year when they won the title. Kobe Bryant was in year four but didn't really play as a rookie.
Father Time will get to LeBron. He has a game that will still be effective as he gets older though. He's good in the post, really improved his shooting, and is a great passer. He will be really good until he retires, but he's been playing heavy minutes since he was 18 with lots of deep playoff runs. He won't be the same that much longer.
Tim Duncan is a good call, but it's highly unlikely any of the rookie class will be able to sniff Duncan's jock.