After reading and considering, I think I can summerize my thoughts. Examining the dumbing down of demons, four difficulties immediately present themselves:
1) Absolute value and consistency. Demons have had their absolute power or CR reduced in Fiendish Codex Vol. 1 Hordes of the Abyss. This reduction is absolute, without respect to how much any individual has been reduced. By one measure, it is fair to say that the cachet associated with demons as Big Bads has been lessened; the are less Big and less Bad. More importantly, there is no meta-setting explanation for why demons are suddenly less powerful - they just are. This is an issue as between editions but also an issue even within just the current edition. There is now an unexplained inconsistency that sees demons suddenly less powerful. Such an inconsistency effecting such iconic creatures needlessly throws a spanner into the meta-setting. Of course, some may rationalize it all away with - “We’ll just assume it always was that way” - which however intellectually vacuous may not be an issue, unless you have been using the demon princes at their former power levels, in which case your game now has need for adjustment.
2) Comparative power in 3rd Edition. In the 3X version of the game, it takes much less time to progress from 1st Level to 20th Level. Indeed, the 3rd edition concept of the “adventure path” thrives on seeing characters rise from 1st to 20th Level in a series of 12 to 14 adventures. A 20th Level character is then not the achievement or rarity it was in earlier editions. It is more commonplace and demons, reduced to being suitable opponents for 20th level characters to defeat, are rendered more commonplace in turn. This runs exactly contrary to how demons have been used and have developed within the D&D mythology heretofore. Demons, as presented, are no more tough than an appropriately advanced monster or player character. Demons place in the D&D mythology as opponents for player characters has been redefined to make them less special.
3) The illogic of lower power demon princes. Reducing the power of the demon princes creates internal inconsistencies when the Abyss is considered. Demon princes are now not that much more powerful than the creatures who they allegedly command and who allegedly fear them. Indeed, a number of other denizens of the Abyss are now intellectually on a par with the demon princes and more powerful, yet we are asked to believe that, without explanation, the demon princes somehow hold onto their positions and keep more powerful, equally powerful and nearly as powerful minions in check. Are demon princes now politicians among their own kind, ruling not through raw power and fear, but by coalition building and alliances among their chaotic evil fellows? Juiblex stumping for votes? Demogorgon proposing a ballot initiative? Grazzt invoking Roberts Rules of Order? The notion of lesser powered demon princes behaving in this way runs in the face of the prior mythology and is absurd on its face. By way of specific example, consider the Klurichir demon from the 3X Fiend Folio, CR 25, said to prey on weaker demons and to be tactically brilliant. Yeenoghu, picking a demon prince, is CR 20. Yeenoghu would be seen as weak by a Klurichir type. If the first Klurichir didn’t vanquish Yeenoghu, then the second, or the third, or the fourth, for Klurichir are not unique individuals but demons types. They will keep coming. Yeenoghu is toast at a lower CR, unless there is a good explanation. There is none offered in Fiendish Codex Vol. 1.
4) Planar mechanics. Demons are the physical embodiment of metaphysical evil and demon princes are the greatest such embodiments. Demons are not just another race, like elves or orcs or even githyanki or mindflayers. They embody all that is quintessentially evil. They are evil given form. What are the 20th level characters who may now reasonably contemplate killing a demon prince? They embody what metaphysical principle? None. They represent what? The successful accumulation of experience, treasure and the acquisition of magic items. And this alone qualifies them to take down the purest manifestation of a metaphysical concept such as evil? There is no logic to this. What is more, allowing 20th level characters to destroy the physical embodiment of metaphysical evil denigrates the very notion of metaphysical evil. It ain’t so bad if a 20th level character, having risen to that station in 12 to 14 adventures, following an “adventure path,” can put down an evil that has supposedly held all in fear for millennia. It simply makes no sense.
Of course, individuals can house rule matters in their home games but such is irrelevant to the presentation as given, as The Rules As Written (RAW). The RAW of Fiendish Codex Vol. 1 is poorly thought out, inconsistent, illogical and without explanation undermines some of the greatest mythology in D&D. Such can be dismissed only by those who either have no appreciation for the planes as they have been developed in D&D or are Wotc apologists or are simply being contrary, refusing to note the state of the emperor’s new clothes. YMMV
The commonly espoused rational that “no one plays epic and most games end at 20th level” in no way addresses the harm done to the D&D mythology and in no way addresses the above issues. It merely creates a justification based on the perception that most players and Dms would find lower powered demon princes “more useful.” Of course, this ignores the fact that heretofore demon princes have been found so interesting by so many and “useful” precisely because they presented the challenge that has now been reduced.
Further complicating matters, the very same authors who wrote the Fiendish Codex Vol. 1, when confronted on the issue of lesser powered demon princes, opine that Dungeon and Dragon will continue to present demon princes as more powerful entities. While a sop, this method of proceeding creates further inconsistencies within the meta-setting.
I would like to have individual(s) responsible for the decision to lower the CR of the demon princes step forward and demonstrate the courage of their convictions, even if they can offer no rebuttal worthy of the name. Who decided that the CRs should be lowered?