You also cannot assume that the synergies of powers do not make up for the loss in bonuses, especially based on math that makes the faulty assumption that powers don't have a strong influence in the combat.
Actually, I think I can. For three reasons:
1) The bonuses that you have been talking about are mostly sporadic in nature. Many of them last for only a single attack or often at best to the end of the PC's next turn.
The penalties I am talking about occur in every encounter. Every round. For every PC.
So, I think the burden of proof has to be that powers do indeed overcome that known penalty, not that they do not.
2) One very important factor here is that WotC put in +4 feats which stack with everything to boost defenses (and you yourself stated were broken) along with a feat that give +3 to hit (after WotC explicitly stated that plus to hit feats and abilities would be rare).
Bottom line: it appears that WotC thinks that the high level math is broken.
3) People who play Epic have indicated how grindy it is. Part of the reason for that is obviously because high level monsters have too many hit points. But, the fact that PCs have -3 to hit and -3 to 3 of their defenses is obviously another factor in the grindiness. If they could hit easier and not get hit as often, they would use up fewer resources per encounter and the encounters would be shorter in duration.
The information we have is not that the PCs cannot handle high level encounters. They can. It's that they hit infrequently, get hit frequently, and have to use many resources and many rounds to overcome the hit points of the monsters. This causes a grindy situation. But because they have so many options and powers, they can handle it. It's just slow and long and resource heavy and grindy.