I previously thought that Epic-tier handled this growing gap between PC attack and critter defenses through the increasing bonuses granted by Leaders and utility powers.
For example: a 1st level strength-based Cleric's at-will grants one ally a +4 bonus to attack. Same Cleric, at 30th level, could to be granting a +9 bonus with the same power. So the fact that all 30th level PC attacks were at -3 relative to 1st level target number could have been compensated by the difference between +4 and +9.
However, such scaling attack bonuses may have been the exception, not the rule: new Leaders seem to give more static attack bonuses.
The Expertise line of feats is an admission that my thinking wasn't what WotC was working towards at all.
And the fact that the +9 to hit typically doesn't occur until the Cleric runs out of most Encounter attack powers, and when the Cleric uses that power, he has a 40% chance to hit in order to give out the +45%. That will only help 2 rounds out of 5.
And the fact that many Epic monsters have buffs and/or debuffs, just like PCs do.