Well, I might agree with this previous to the fix to increase monster damage. Now, I don't believe it for a second.
The changes to monsters definitely changed things, but... in my experience, it meant that you could actually run at-level encounters as average encounters, rather than having them just being speedbumps, and needing Level+2 or Level+3 in order to be a standard encounter (and even higher for actually challenging fights).
So... yes, the Expertise feats are needed for a group that only fights encounters 3 or 4 levels above them. I don't feel they are needed to 'keep up' with the normal expected challenge of the game, however.
The simple math does show that a PC will 'lose' 3-4 points of attack bonus over the course of 30 levels. However, that is entirely absent of the rest of the context of the game - such as the fact that they quadruple the number of encounter powers and daily powers they have, and gain even more encounter powers, items, along with special abilities from paragon paths and epic destinies (and now themes as well). I've found that you'll make up about half that bonus based on different builds and conditional feats, and the other half tends to be made up for in the increased potency offered by your greater array of powers and options.
Now, others disagree, and... fair enough. But
with the Expertise in play, by Epic levels, my experience was that PCs almost never missed. It didn't mean they hit 'as often' as they did at level 1 - it meant they hit nearly all the time.
This was, admittedly, with a somewhat optimized group (though certainly not at the very extreme of such things.) And I do get that the balance needs to be aimed at the 'average' PC. But the math that Expertise is supposedly based on isn't aimed at an average PC, it is aimed at a PC who completely lacks a Paragon Path, Epic Destiny, Theme, feats, items, powers. It only really works in the absolute void of every other relevant aspect of the game, and once those other elements are added in, I think it
unbalances the math too far in the other direction.
In my opinion and experience, at least.
Because Controllers really have very limited control, and every class (shy of a lazy Warlord) except Striker does little damage, and because monsters hit so much harder now, everyone is trying to tweak out every tiny little bit to hit and to damage whenever possible. It's become an arms race of getting the absolute best stuff, not much different than picking the absolute best cards in Magic to ensure your ability to be competitive.
Is this something that has only just developed? I'm not saying I haven't seen such behavior - I definitely play with one group that often takes that approach - but it was just as true at the start of 4E as now.
It is true that the challenge tends to adapt to the PCs - I view the boost in monster damage as getting monsters 'back in line', though, so that the proper challenge level they provide is actually as expected.
If one looks at it objectively, first WotC handed out Expertise, defense feat boosts, and masterwork armor changes to the PCs (AV and PHB2) to balance out the "to hit" math. Then when the game became too easy, the concept (that many players did mention early on) that the monsters did not do enough damage was addressed (MM3), attempting to balance out the "damage" math. This shifted the balance of power back to the monsters. WotC then introduced more surgeless healing / temporary hit points (now that damage actually mattered), simpler and stronger Essentials PCs (not necessarily overall, but with respect to an average attack), and the ability for PCs to use any number of item Daily powers at any time (a major boost in versatility). This shifted the balance of power back to the PCs. WotC then introduced the concept of common, uncommon, and rare magic items in an attempt to shift the balance of power back to the monsters (this was done at the same time as Essentials, but it's pretty obvious that Essentials classes and feats are in many ways stronger and/or more versatile than many core classes and feats, so it looked like an attempt to balance Essentials right out of the box, which was a good thing). Then this year, they handed out Themes which have the potential to increase the versatility of the PCs. Again, power shift towards the PCs.
Hmm, I'm not sure all that is as 'cause and effect' as you feel. Even before expertise, the feeling was that monsters were low-damage; it wasn't that this made them 'too easy', just as the supposed math gap didn't actually make them 'too hard' - they both were just seen as contributing to the dreaded 'grind'.
Surgeless healing actually got toned down,
after monster damage had gone up. I don't know if temps have actually become more plentiful, so much as the result of newer classes needing new design space to fill out, rather than retreading the design areas we've seen before.
The change in magic items - removing daily powers and adding a rarity system - happened
simultaneously. One wasn't an attempt to boost PCs, and the other an attempt to tone them down. They were both rooted in completely seperate issues from balance - the perceived flavor of items, and their use in the game.
Similarly, Essentials characters are not by default any better or worse than existing PCs (despite that many folks have made both claims). They have their differences, but I don't think they've shifted any fundamental power level - aside from, and this is one genuine change, the shift in power level for feats, which did definitely rise with Essentials. (And, largely, ties back into the entire expertise issue.)
Honestly, I haven't seen any fundamental back and forth rebalancing as you have described. Changes have definitely been made, but I think they have had a variety of causes and reasons behind them, rather than each one being somehow a direct response to the change exactly prior to them.
So I don't think we can just look at the to hit math gap as one game element out of sync. There were several out of sync elements, WotC has addressed them over time, and it has resulted in major shifts in many things including what types of roles are found at tables in which ratios. We only have one leader in our Paragon campaign out of 5 to 9 PCs (depending on who shows up) because even in extremely difficult encounters, we really don't need a ton of extra healing (we also only have one leader in heroic out of 7 PCs). The strikers tend to kill the foes quickly, the controllers tend to debuff the NPCs (we have one who gives the NPCs anywhere from -4 to -7 to hit the PCs, so even defender marks/auras become a bit moot), the defenders stand around taking a significant portion of the damage, and the leaders are mostly there as an emergency healing/control stopgap, so many leaders are not needed.
I'd say that the lack of need for leaders may be a good example that balance
has been compromised. I'm not saying the game
should require a group to have every role covered in order to succeed. But Expertise has made it so easy to focus on damage and Strikers, and simply assume you can kill the opposition first.
At this point, I'm not sure there is any easy way to remove the expertise feats from the game (though I have been having success in my current campaign doing just that). But everything I've seen still tells me that they were put in place to fix a perceived problem that was not actually the issue they thought it was, and they've instead ended up unbalancing the math more than balancing it.