I think it's case by case, Rose has always been a PG, he played it in HS. Russ has done a decent job of converting to the position considering he's basically doing it on the fly. I think hyper athletic guards can get injured and come back from it, Baron Davis tore his ACL before he got to the league and still managed fine.
Also, Chris Paul was ridiculously athletic when he first came into the league, he even had a higher vert than Westbrook, he's just 3-4 inches shorter than Rose/Westbrook. I remember he banged on Mr. No Post Moves Dwight. And I think it's inevitable that we're going to see more and more natural PG's that have crazy athleticism, that Seventh Woods kid is a natural PG and he has Westbrook/Rose athleticism, so the line is going to get blurred.