It's probably worth reiterating that it doesn't matter if Expertise is "necessary" or not. It exists. If you have a +2 - +3 attack bonus available for a feat, especially if it also gives invaluable things like staff expertise, then people will take it.
So, either the feats are a math fix _or_ they're overpowered, but there's no real middle ground in which the system works hunky dory whether you choose to take them or not. They're a blemish of _some sort_, whether it's systemic or just confined to them.
But they do (assuming for the moment the bonuses are truly 'missing'). Even if just encounter and daily powers grant the scaling bonuses, there's more than enough of them to last through the number of encounters you're likely to have in a given day.
Even judging from what I've seen in our first level party we won't ever have trouble hitting monsters in the epic tier: Our two leaders grant at-will combat advantage, and a +2 power bonus to attack rolls, respectively. So, we're already well ahead of the game.
In my general experience (several hundred sessions ranging from 1st - 23rd level), these types of bonuses are actually most common at low level - you do less fights per day (2-3 in heroic, 5-7 in epic, frex), more of the basic at-will powers give bonuses to attack, which people largely stop using, etc. As you get higher level, people tend to do harder control powers or attack granting instead, in my experience, such that you still see bonuses, but they actually go far down in quantity and regularity. Combat advantage comes from powers (daze + stun + prone) much more commonly, but on the other hand flanking (and PCs actually standing near each other in general) is less common. At least among those who want to survive all of the blasts and bursts coming their way
Likewise defenses: Marked enemies get a -2 to hit everyone but the one who marked them and most pcs already have ways to become invisible (though all except one can only do it once per encounter) for an additional -5 to hit.
Depends on the party - usually the way it works if you actually have some near unhittable people, and some near autohittable people. Like, you might have a 22nd barbarian with Will 27 (monsters are attacking at +25) and a Will 43 Chaladin (w/ +2 to all defenses until he takes damage), both in the same party.
Marks also trigger less often as bursts become more common, or if they do, it's for situations like the above barbarian vs. paladin choice.
Difficulty and chance to hit don't have to have anything to do with each other.
Personally, if attack bonus were decoupled entirely from everything but level and maybe a basic class accuracy, I'd be fine with it. That way you could stop worrying about ability score bumps from epic destinies and races, certain feats, certain magic items, and all the rest.
