Descending though is a particular gift. Have you ever raced with pros on a descent?
It isn't that simple. Going down requires a self-suredness at 80kph that not everyone possesses. Even among the pros.
I have and I can tell you that no matter how good you are uphill, if you can't descend then you have serious problems. Now, at that level, it becomes even more critical. It isn't that he can't go down fast...just not fast enough. They ride away, or you crash. This is a different skill, which has nothing to do with watts, or cardiovascular capcity, but space-time reactionary skills that either you have, or you don't. He doesn't and that was fatal, or nearly so.
You'd think that after all the suffering and the grit that this should be enough to carry you through, but alas then there is the descent, which is unforgiving and ruthless.