The reason the Olympics were held in Korea during the dates you mentioned and not Summer proper is that most of the annual rainfall in Korea falls between June and September, ditto Japan which has to contend with typhoons and tropical cyclones. As for Australia September and October are definitely not Summer months. Those 3 examples you gave are exceptions to the rule; the Summer Olympics are just that, Summer games.
As far as I recall we've had temperatures in the mid to upper 30s in October and it is still quite humid then. Ultimately the success of events such as the Olympic Games and the World Cup are gauged by popular attendance. To date most international events held in Doha such as the ATP, Moto GP, PGA and others have not been well attended by the general public. And the sight of mostly empty or semi-empty stadia is the last thing major sports organizations like the IOC and FIFA want to see.
It'll be interesting to see what the Asian Games are like and how much of the attendance will NOT be the scores of school children who will be drafted into the event or people coming because free draws on cars and such are held to encourage attendance.