I personally doubt that more people are playing epic games than than are those using psionic material.
I also think that when you see 'epic' material, it suffers a stigma (not an always wrongly presumptive stigma) of treating high level games as not epic but [booming voice]EPIC!!![/booming voice] that ends up being more munchkin and 'PCs go on a godkilling spree without any plot beyond starting at A and going to Z with their +395 godkilling swords of epic killing power.
It's rare that I've seen high level games encompass depth of plot and some realism beyong 'the PCs are the heros and thus anything can be killed, and those beings will wait around as big targets till the PCs can take them on in their epic level extraplanar dungeons'. Obviously that treatment is a bit of hyperbole to some extent, but it, and its ilk drives me up a wall.
That said, the level of writing in Dungeon of late (which got me to subscribe) alleviates any risk of an epic level adventure path falling into those frequent pitfalls of 'epic' adventures.
With regards to doing a Blood War adventure path. Don't treat it as something that you can stop, or that you even want to stop. The Blood War predates the Gods, it prevents the Lower Planes from uniting, it is both a blessing and a curse for the upper planes, a sinkhole of morality and the foremost expression of evil falling in upon itself.
However the seeds are set if they wanted some plot(s) to have epic level PCs have a hand in, and have a realistic chance of actually having any effect upon the Blood War:
1) Grazzt wants to end the Blood War, temporarily, in order to have a united lower planes devour the upper planes, and after that in his mind the Tanar'ri would then once more turn upon the Baatezu. If he managed to have any sort of chance to pull this off, that might make an interesting plot arc. Obviously other groups would oppose him (the upper planes, the Yugoloths since they started it and they damn well want to be the ones to end it, the Rilmani, etc and would make for some bizarre bedfellows).
2) The Yugoloths at some point, or their makers the Baern, will want to end the Blood War if they feel that it has served its ultimate purpose. At that point they'd intend to obliterate the Upper Planes with a subjugated Abyss and Baator serving as cannon fodder under their rule. This won't happen likely at any point in the immediate future, but perhaps a splinter group within the yugoloth heirarchy, or perhaps a renegade Baernaloth (in the same tradition of Apomps the Triple Aspected) felt differently, and felt the need to accelerate/end their racial handling of the Blood War as a massive experiment and turn it into a conquest of the Upper Planes.
3) Perhaps a group of Celestials decide to take an active roll in the Blood War and they risk turning the lower planes against the upper (which happened once before, ended terribly for the celestials, and is largely why they no longer take a part in it). Perhaps the PCs would be the ones to stop them from doing this, perhaps finding that they were colluding with other splinter groups in the Lower Planes, etc.
FWIW, I ran a campaign up to about level 30(see storyhour #1), ultimately involving the PCs in a conflict* versus the Oinoloth and a cabal of Baernaloths. Took about three years from start to finish, and didn't fall into the trap of PCs gone wild killing abyssal lords in one on one combat, but also not making them insignificant mortals without a hope of changing anything in the War Eternal, which is normally the case for anyone really involved.
* this was not a direct 'roll initiative versus the Oinoloth' conflict because they'd have gotten crushed like bugs. Archfiends aren't just freaking bigger monsters to encounter in Abyssal dungeons to kill, they deserve more intelligent and in depth treatment.
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