These are good points about where Epic balance should be.
That said, who knows what's out there. I don't know if there are plenty of 70, 20/x4 crit enemies in Epic. If there are, I am doubtful Turbine would have made those damage profiles if they anticipated players wouldn't have 100%.
If Fort were modified and the 70, 20/x4 trash mobs do exist, those damage profiles would have to be brought in line. As you said, it's deeply unfair to spellcasters.
But in a mundane world of 70 20/x2s, though, it's less terrifying. You're looking at a 5% increase in damage per 20 swings. With a 5% reduction, you're looking at crits of 132.
I'd consider shaving -10% to -15% off Epic damage above and beyond whatever the crits require. The numbers really start looking less absurd, and the spikes kinda start looking even fun; those 800 HP players who would whine about Turbine making an "easy-button" will still have their triple-digit crits to pose a challenge.
Keep in mind the whole system should be designed to make some level of effective Fort possible for most players, most of the time. The extremes having full fort or zero fort shouldn't be the mean.
How would that work? Just ask yourself what conceivable anti-seeker bonus would be "good enough" for minimally effective fort for most players that try without excessive twinking. Pre-Epic, I think +20/+25 would be in the zone; in Epic, you might need an item with +35 or +40 -- or with the direction they are taking Epic to-hits, +50 may turn out to be reasonable.
This would not capture everyone under its umbrella. Turbine will have to design content to be survivable for low fort characters. Barbs have such penalties to AC they probably wouldn't even try for Fort; but human barbs may become a little more popular due to the ability to snag a toughness feat. Meanwhile, a rogue should still be able to dump defense and push HPs to beyond the 500s if it wants, but it will be more difficult to get the same predictable survivability that the rogue would have with a more balanced offense and defense. But a rogue won't get hit as often, so its up to the rogue. Choice is good.
I actually think this is a good reason to attempt to rework the current Fort system. At present, easy Heavy Fort disproportionately benefits Barbs and DPS melees. Those classes are designed to soak massive damage with HPs and DR, and Heavy Fort relieves them of the largest share of that damage. They are at the most at-risk of crits, because they take the most hits. But they are crit-immune. Imagine any other level 9 item that increases the effective red bar of the 500+hp melee by anywhere from 5%-30%.
As for casters... I won't get into all the opinions I have about the role casters should play in Epic content design. They lack their best defenses, which traditionally has never been Fort but rather their offensive spells. In the absence of removing mob immunities, having access to damage mitigation is crucial for these casters. Rather than through items, though, I'm a fan of accessing Fort through spells like Iron Body (with Still Spell metamagics, of course) and abilities; that way, these "vulnerable" classes will have effective fort without creating easy-fort items for the melees to hoard.
Having written all this, though, I'm pretty sure the %-debuff system Eladrin spoke of is the wave of the future. Which has its own benefits, but mostly for no-defense, high-HP classes that can absorb the damage. I'll just wait til we hear more about it.