Let's just look at this and see if it is a problem...
Monster ACs rise by 1 / level (100% per level). Let's now track the increase in PC attack bonuses assuming the following:
* Key attack attribute started even and rises at every opportunity.
* Improved magic weapon found every 4 or 5 levels (4, 8, 12, 17, 21, 26)
Bonuses thus improve:
Level 01: Starting
Level 02: +1 (even) (even level)
Level 03: -- (behind 1)
Level 04: +2 (even) (item and even level)
Level 05: -- (behind 1)
Level 06: +1 (behind 1) (even level)
Level 07: -- (behind 2)
Level 08: +3 (on par) (item, even level and attribute)
Level 09: -- (behind 1)
Level 10: +1 (behind 1) (even level)
Level 11: -- (behind 2)
Level 12: +2 (behind 1) (item and even level)
Level 13: -- (behind 2)
Level 14: +2 (behind 1) (even level and attribute)
Level 15: -- (behind 2)
Level 16: +1 (behind 2) (even level)
Level 17: +1 (behind 2) (item)
Level 18: +1 (behind 2) (even level)
Level 19: -- (behind 3)
Level 20: +1 (behind 3) (even level)
Level 21: +3 (behind 1) (item, attribute, paragon path (not all pps have a +2 to attribute, but the most common choice does)
Level 22: +1 (behind 1) (even level)
Level 23: -- (behind 2)
Level 24: +1 (behind 2) (even level)
Level 25: -- (behind 3)
Level 26: +1 (behind 3) (even level)
Level 27: +1 (behind 3) (item)
Level 28: +2 (behind 2) (even level and attribute)
Level 29: -- (behind 3)
Level 30: +1 (behind 3) (even level)
You basically jump around between 1 and 3 behind - and I was assuming you'd find magic weapons/implements a bit later than most people do. Once you begin to factor in the increased access to bonuses to hit (greater positioning control, more powerful leader abilities, more powers with + to hit built in, etc....) you'll see that this isn't really a decline at all.
A starting PC on the low end will often have an weapon attack bonus (versus AC) of +2 proficiency and +3 attribute bonus for a +5 bonus. The average soldier (the hardest role to hit with AC) of their level has a starting AC of 17 (per DMG) - requiring them to have a 12 to hit. An elite has 2 higher AC, which brings that up to 14 needed. [If you start off on the high end, that drops by 3 (20 attribute and +3 proficiency) to 11 (or 10 for some rogues).]
If you factor in a +2 bonus for leader powers or flanking, you can get up to needing a 12 instead of a 14 if you're on the low end of AB.
Even if your 'AB' falls behind by 3, that character with the low attributes is still hitting on a 17 without bonuses - against elite creatures of their level.
Now, you do start getting in trouble if you have lowish stats, your proficiency bonus is low, you're at one of those '3 behind' levels, you get no bonuses (flank/leadeer/etc...), you're facing an elite AND that elite is a few levels above your level... but that should be a hard encounter.
All in all - the system works.
And if that argument isn't persuasive, think of it slightly differently. Assume that you're starting with an AB situation that is too high compared to monster defenses - and that it will drop to being in line with time.