Lots of interesting ideas, here. I notice that some people who seldom post feel strongly enough to comment as well.
Obviously, you can't please all of the people all of the time, and there will always be dissent on this issue. There are any number of competing canons, and players' viewpoints often border on fundamentalism. That said, for me

:
1) The thing that bugs me most is consistency. I would have liked to see a level playing field drawn between the Draconomicon articles in
Dragon, and the
Fiendish Codices (we can assume that Devils will recieve the same treatment?). This strikes me as pretty basic. In other words, updated (and in some cases, powered-up) BoVD stats.
2) For those who suggest that advancement lies in the hands of the DM, I agree. The problem is that hand-crafting a demon prince takes an enormous amount of time.
3) It speaks volumes about how poorly supported epic-level play in general is, and the default assumption is Lvl 20 tops. This is a shame - I run an epic-level game, and I'd like more material to draw on. But if WotC did their research - and surely they must have done - it puts me firmly in a minority camp. I don't anticipate any new supplements in this area. I guess I just have to suck it up, and do any modifications myself.
4) My resigned appeal to whoever is writing the second (or third?)
Fiendish Codex (James Jacobs? Erik Mona?) would be
Please devote a respectable amount of space to ideas about how advancement can be made flavorful. This might include notes on adding HD or class levels, new special abilities, upping SR, a revamped Paragon template (or something less extreme), divine ranks, integrated caster levels, etc. etc. Although I think accurate balance beyond CR30 is close to unattainable, I think we can ball-park it within a CR or 2.
It remains my hope that the listed CRs of all of the demon lords in FC 1 are simply inaccurate and too low. By about 6 to 8. Less work for me, that way.