If one goes back to AD&D and looks in the DMG, items were handed out because the DM wanted to, or because the DM rolled random dice for treasure.
If rolled random, the odds of getting a +1 sword of some type out of a single magical item roll were a little over 5%.
The odds of getting a +5 sword of some type were about 0.3%.
It wasn't a system of PCs getting a +1 item at level 3, a +2 at level 6, etc.
It was a system of the either the DM controlling what was handed out, or the system itself attempting to minimize PCs getting uber items.
Given the OP's question, my first thought is that inherent bonuses do not have to be +6 max, nor do magic items have to increase to +6 either.
There are problems in the game system such as the fact that several ability scores for most PCs only increase by 2 overall for a grand total of +1 while others can increase by 10 for a grand total of +5.
If the ability scores increased less frequently, that would be less of an issue. If all 6 ability scores increased every time, that would be less of an issue.
So let's redesign the entire system:
1) Monsters get +1 to hit and to defenses every level.
2) PCs get +1 to all ability scores on levels 5, 10, 15, 20, 25, and 30. For simplicity, this could be +2 to all ability scores on levels 10, 20, and 30. The absolute largest delta on a defense (without class or racial abilities) is 6 using point buy. For example, a PC that has a 20 Str, 9 Wis, and 9 Cha. But more often, the delta would be 4.
Increasing ability scores appears to be a sacred cow now. It's not really needed, but some people will throw canaries if it is dropped.
3) PCs get +1 to hit and to defenses 4 levels out of every 5 and there are no inherent bonuses. +1 at levels 2, 3, 4, 5, 7, 8, 9, 10, 12, etc. Total: +24.
So, the total automatic bonus is +27 for PCs. +29 for monsters.
Magic items range from +1 to +3. DMs are encouraged to hand out a few +1 items at heroic, +2 at paragon, and +3 at epic, but it is not required. In fact, a PC might have a +3 sword at level 25, but only have a +1 neck item and no magical armor whatsoever.
An artifact or relic can be +4 (e.g. a Holy Avenger might be a relic), but these items should be super rare and should strongly affect their owners/wielders.
There are no non-conditional feat or Class or Paragon Path or Epic Destiny bonuses to hit or to defenses. A bonus to hit should have conditions attached to it such as "only when the PC has combat advantage" or "only when the foe is bloodied" and these should be rare. Most bonuses should be given by Leaders via powers and bonuses should almost always be in the +1 to +2 range and only on Dailies be based on ability score modifiers (i.e. +5 to hit).
Finally, magic items should have more umph than just +1 to hit and +1 damage. There should be other benefits of having any magical weapon or armor. For example, maybe all magic armor gives damage resist 3 per plus automatically. Players would really work hard to acquire magic armor if it did that and damage resist was difficult to get otherwise.