Here's a simple explanation. Those feats are parallels for the equivalent Epic feats from 3.x. People wanted feats to bolster their stuff, Wizards decided to do so. There's no evidence presented that it's a 'fix' when Wizards has shown -multiple times- that they're willing to institute fixes with errata.
However, just because I haven't seen an explanation for the Expertise feats that doesn't involve "math fix" doesn't mean there isn't one. If there were such an explanation, it would certainly cast doubt on the "huge untyped bonus from feat==math fix" line of reasoning. 2) Got any in mind?
Popular demand? Look at all the Char-Op. It's -all- based on maximizing to hit, and maximizing defenses. Char-Op players -are- a signifigant number of players of D&D, and perhaps Wizards wanted to toss them a bone?
That's the easiest explanation.
As I said before, it's also very hard to find any evidence that FRW attacks were intended to scale better on to-hit and AC attacks were supposed to scale better on damage.
I'll be honest, I don't see a lot of looking from the 'math-fix' camp. Perhaps I'd find a lot more weight to their arguments if they actually bothered to look behind the to-hit number.
It's really difficult to find something with your eyes closed.
3) Why would the DMG be completely silent about what would have been a major (intentional) factor in the design of higher level monsters?
Let's take a look at this from a different angle.
If the to-hit was supposed to be 50/50 (ish) across all levels, then you'd expect damage to remain proportional to character hitpoints.
Let's examine this 15 Con fighter again. I like him. He's rugged and has a cool name: Smishface. He has a neat cape too.
At level 1, Smishface has 30 hps.
A monster, according to the DMG on page 184, can (if he is using a high-damage normal attack) be expected to have a damage of 2d6+3 to hit, or 10hps. This gives an effective damage-per-hit of 33%.
The interesting thing is we can go level by level and calculate effective threat-per-hit according to this table.
1 Damage = 2d6+3 = 10, hps = 30, threat = 33.3%
4 Damage = 2d8+4 = 13, hps = 48, threat = 27%
7 Damage = 2d8+5 = 14, hps = 66, threat = 21%
10 Damage = 3d6+5 = 15.5, hps = 84, threat = 18%
13 Damage = 3d6+6 = 16.5, hps = 102, threat = 16%
16 Damage = 3d8+7 = 20.5, hps = 120, threat = 17%
19 Damage = 3d8+7 = 20.5, hps = 138, threat = 14%
22 Damage = 4d6+8 = 22, hps = 156, threat = 14%
25 Damage = 4d6+9 = 23, hps = 174, threat = 13%
28 Damage = 4d8+10 = 28, hps = 192, threat = 14%
So, the effective threat to hps of a single hit goes down as the monster goes up in level.
So, for the monster's attacks to have equal effectiveness over levels, the chance to hit -must- go up as the monster gains levels.
Therefore, the concept of 50/50 chance to hit is intended is either incorrect, or broken. However, we've proven that monsters don't have 50/50 to hit. Which means that expecting 50/50 is a falsehood.
It doesn't even get mentioned in the book.
Now, then, let's look at the FRW vs AC debacle.
To hit FRW and to hit AC should according to the same page be within 2 points of each other for a monster. Using light armor as a base (as heavy armor is balanced with light armor in mind) we can see clearly that:
For cloth wearers, the disparity between AC and Reflex (best save for light armor wearers) is 0 at heroic, 1 at paragon, and 2 at epic. For leather, it's 2/3/4 and for hide it's 3/4/5. Everything else is derived from statistics that affect both AC and FRW.
Where the disparity is less than 2, there's no problem:
Cloth wearers, Leather wearers at heroic.
Where it is greater than 2, there might be a problem:
Leather wearers at paragon, hide wearers.
So let's look at the difference. It ranges between 1-3 points of difference...
...so for any feat to be a 'fix' it would have to be between 1-3 points of difference.
In other words....
...
Your 'fix' was already included in the PHB1. It's called Lightning Reflexes, Iron Will, Great Fortitude.
For what it's worth, this logic is wrong. It is not the case that FRW boosts become especially valuable because FRW scales poorly. I could think of some reasons why said fighter might pick up a +6 neck slot item before Armor or Weapon, but this isn't one of them.
If these boosts aren't valuable because of poor scaling, then they aren't exactly fixes for the problem, are they?
You can't have a case where they are logically fixes for the problem and yet, are not valuable -because- of the problem. This is a logic fail.
I counter with this: If I believe that my FRW is lower than they should be, and that my other defenses and offense is just fine, then I'm more likely to shore up my weakness, even if it is by a little bit.
EDIT:
I'm going through the Monster Manual, and doing some quick comparisons between equivalent level monsters' FRW attacks and AC attacks. So far, just looking at Terrasque and Red Dragon, the pattern of 2 off seems to mostly fit. Comparing Crownwing and Storm Gorgon (both brutes of the same level) also shows a 2 point difference between an attack vs AC and vs FRW.
It appears that 'these feats are an obvious fix' isn't obvious when looked at with any level of rigor.