Yes, I know they do release errata that clarifies rules or fixes some broken power, but doesn't address larger fundamental flaws in the system. Many of the math holes are plugged with stealth errata "feat taxes" in the form of feats from supplements which are not free.
Where's the errata for lagging attack bonuses and defenses so the expertise an defense feats aren't needed? Where is the errata for reducing monster hit points to a more reasonable level (and maybe upping their damage to compensate), so combat isn't a long drawn out grind through a sea of monster hit points? Where is the errata that makes magic items magic again? Where's the errata that lets non strength based characters hava a decent basic attack? I could go on, but you get the picture.
It quite obviously wasn't a feat tax because it was Deva only. You can't possibly believe that the designers tried to patch the Swordmage by introducing a feat that only applies to a single race. That isn't a feat tax, that's just a typical feat benefit akin to Enlarged Dragon Breath or any number of other feats. It's pretty clear to me that at the time that the Deva feat was written, Swordmage Warding wasn't considered a serious issue.
If you're complaining that the designers should have been faster in instituting the errata, then you may have a point (I really don't know what kind of red tape errata has to go through to get approved, so I'm withholding judgment).
Saying that the -8 just begs a CdG is like saying that any cloth wearing class (ie, the Wizard) just begs the DM to CdG it when it goes down because it's in effectively the same boat. That -5 is a pretty huge penalty, almost guaranteeing a hit regardless of what kind of armor you have. If the DM wants to CdG someone then he will, and the thing that's likely to save you is the damage roll, not the attack roll.
You really don't see how not losing 3-5 AC while unconscious on top of the 5 you lose by default would be somewhat of a deterrent to the DM, who instead of CdGing you while you're down, by a lot, might decide to hit someone who's still up and doing damage instead? If you're down but only by 5 like every other Defender would be, say you're L2 and you've got 21 AC normally with ISW, you've still got 16 AC and there's a somewhat reasonable chance of you actually being missed, DM will probably go for the conscious guy still doing damage who has, say, 17 AC. If you're down by 9 due to losing your warding, you've got 12 AC and chances are really good that he's going to hit you period, giving him no real reason to look elsewhere instead of potentially getting rid of that annoying guy who's usually a big bag of too much AC and getting in the way of killing the squishies. What anyone else's AC is while unconscious is irrelevant in that situation. DM's only going to care that right now you'd be so very easy to get rid of.
So why did your post state that WotC would never own up to a mistake?
They may fix rain of blows, but have they ever even addressed the issue of the grind due to monsters with too many hitpoints?
Actually, they did. Page 133 of the DMG2 addresses this issue of grind, recommending lower hp for solos and lower defenses for both solos and elites.
If not every group experiences grind, I'd say it's not inherent to the system, and thus not something that a rules update can fix. Also, IIRC, combats lasting 45 minutes to an hour really is the norm for 4e. (It was the norm for my group in mid-level 3.5, too, fwiw; 1e on the other hand is a ton faster.) If you're not happy with the length of combat, it's a matter of preference, not an inherent system problem which would merit errata.The wonkiness of 4E math can be somewhat overlooked since it at least can be mitigated with feats(even if they seem like feat taxes), but the "the grind" is just horrendous and needs to be addressed officially because it really hinders enjoyment of the game.