Later during his career Greene himself was talking about fine wines and getting better with age, although there were faster sprinters at that time already. 
28 is still quite young for a sprinter. And an athete who’s been running most of his life, why not still give it a shot and see what’s gonna happen? I doubt if he can challenge Bolt but sub-10 would be impressive enough after a long break. It would be extremely interesting to see him focusing on the 110H.