Yes, Thune is the best candidate at the moment but any 'serious' candidate would win easily.
Nobody is taking down Obama so easily. With the charisma he's got and the media being on his side, any solid Republican would still have a very hard time, even if the economy doesn't improve. The electoral map doesn't help the GOP either now.
No, Obama is black, the media isn't on his side, and the map still favors Republicans heavily. It was a fluke Obama was elected in the first place, and the charisma is the wrong kind, alas. Oh, and all signs point to the economy remaining very bad for many years.