You seem to be discussing this in the context of solos. I think it's pretty well known (see the thread:
http://www.enworld.org/forum/d-d-4th-edition-rules/253587-d-d-solos-just-not-threatening.html) that solos are poorly designed in that many status effects are too effective against them, and at higher levels characters have access to many of these effects, making this a significant problem.
Additionally, AC already scales pretty well at epic levels and is by far the most attacked defense. As I have said many times, there are stronger tests of whether the game's math is off (in the PH) than whether the game becomes too easy at higher levels (edit: of course, I don't endorse the PH II feats as a good way to fix these problems).
In particular, do you think that attacks against FRW become weaker over time relative to attacks against AC in a way that compensates for the better scaling of FRW attacks? I maintain the answer to this is “no.”