A couple of things:
1) Michael Johnson is mentally challenged, and although he was an amazing athlete, he should be disregarded as a commentator.
2) Anybody who doesn't think that Bekele could run under 50 for a 400 is an "idiot" and a "moron". Bekele is one of those distance guys with "pop"--not only does he have the speed endurance to run the last 350m well, he also has the jump to get out of the blocks and accelerate to speed well, and he is very compact making starting on the turn easier for him, and he is very accustomed to running turns at that.
3) Ventolin is an "idiot" in so many ways. When it comes to sprinting, especially the 400 which is really a hybrid event that is more of a run than a sprint, his comments should be disregarded. He assumes some sort of consistency among performances, when sprinting is a totally different activity than running, and when the two do not equate in any sort of linear fashion.
Bekele is one of those few guys who, IMHO, could come very close to sprinting, as well as run very well. Jeilan is the gold standard for this. Bekele could actually drive the first 25m of the 400m, unlike, say, Rupp. If the 400 was characterized as entirely a running event with the acceleration contribution being negligible, then maybe comparisons to other distances would be appropriate--but it cannot. Acceleration is somewhat important, and I think Bekele has some of it.
4) Sorry, but I just cannot believe that Symmonds could beat Bekele in an 800m race--I don't know exactly why, I just can't.
Who would win a 600m race? I have no idea. I think they should do it as a charity event.