More like 10 points higher. A PC should have no problems getting his AC to almost +50.
I'm having trouble getting your +50. I get +45 featless in godplate(10 base, 15 level, 14 armor, 6 enhancement) and +46 featless in hide(10 base, 15 level, 5 armor, 10 ability, 6 enhancement). Those are top-of-the line. I'm not sure it's reasonable to assume that wizards and rogues will have the str and con(and feats) for hide, warlocks will maximize their int, or that wisdom clerics, two-weapon rangers, and warlords will be ready to take plate. Let's say 46 is "average", given the availablility of shields and AC-boosting feats.
If we assume a PC chooses a race with +2 to both his key stats, starts with base16, and goes demigod, he ends up with 28 in both stats at lv30, and say a 12-14 in the 3rd stat. So his defenses are +40/+40/+32. If you want, you can add another +2 with the appropriate defense-boosting feat.
My calculations suggest this doesn't include class bonus, which every character has. So we're actually looking at 42/40/32, 40/40/34, 41/41/32, or 41/40/33. So, difference of 4-6 vs AC for the high ones, and well, okay, lots for the low ones.
A typical lv30 foe has an attack roll of +33, meaning that he will never miss vs your weakest defense, and will hit your strongest defenses on a 10 or more (~ 50-55% chance of success).
About a 50% chance of success is a good thing, means that balance is about where it should be. Never misses on the weak defense is a bit odd, but we'll get to that in a minute.
Conversely, if you focused on all 3 stats evenly, you tend to end up with a defense spread of ~ +37/+37/+37. So the enemy still ends up hitting on a roll of 5 or more, not factoring in combat advantage.
Again, class bonus makes this a little less awful, but it's still more awful than focused.
Seems like you are still screwed either way. Focus on all 3 defenses evenly, and the foe will still be able to reliably overcome them all. Focus on 2 defenses at the expense of the 3rd, and you may or may not be any better off.
See, this is where I worry. With the spread, a character is in trouble if the monster attacks any non-AC defense. But if we drop one, the character is only in trouble if the enemy can target that defense, on that character, at-will.
The Tarrasque can target reflex at-will for 1d12+16 and prone, and fort on a recharge for 3d12+16, push, and prone. Neither of these is nightmare scenario, even if they are auto-hits.
An ancient red dragon can target reflex with a number of damage powers, each of which does less than 60 points of damage, although one(a recharge power) attaches 15 ongoing fire damage. The dragon can target will with an encounter power for a one-round stun, and fortitude as a secondary attack on one of the reflex attacks(so it has to beat at least one good defense). None of these are nightmare scenario on auto-hits.
Orcus has two attacks that target fortitude, both recharge powers. One does 2d12+12 damage, the other reduces the target's hit points to 0. That last one is pretty rough, but it requires Orcus to target, with a melee attack, a character with low fort defense. Many such characters will be going to great lengths to avoid being in melee range of Orcus, and being hit with that, while painful, is not game-ending for 30th level characters. With epic destiny powers and features, they are almost certainly still in the game.
So where exactly is the nightmare scenario? What is the monster power that breaks the game when it auto-hits? I'm not seeing it.
I suppose that it might be expected. Since foes only get to make 1 attack/round, I suppose it is in their best interests to want to hit as often as possible. Though this seems to defeat the purpose of even bothering to boost your defenses, if the foes are still going to hit regardless of your best efforts.
The foes are only going to hit sometimes when they're taking on your
best efforts. They get auto-hits if they target your
worst efforts. And, except in highly unusual parties, different characters will have different weaknesses, so only some characters are free targets to any given power. The rest of the party is ready to go about business as usual.
Is there anything amiss with my observation? The designers sure were not joking when they said that the +X items were crucial to your PC's survival...
Yup, you do have to keep some of your defenses up. If the monsters can auto-hit with
all of their attacks, the PCs are in serious trouble.