I'm curious if you consider the numerous problems people had as far as legitimately challenging their PC's once they got in paragon and epic levels a form of "Monty Haul-ism"? Now traditionally a "Monty Haul" campaign has been when a DM handed out too much treasure and well pretty much the PC's became too powerful. Do you believe the rules can do this as well (if they place themselves in charge of this aspect as opposed to the DM?), and if not what do you think is the primary cause of so many people finding that they had to drastically exceed the game's encounter guidelines in order to offer any type of challenge to their players in these levels? I can kind of understand epic levels since I consider thos a special case (though I do think the math and expectations should still hold up)... but in paragon level should there be this type of disparity?
I think I conceptually understand what you're meaning. However, I'm not really sure if I'm fully aware of the issue that you're depicting. I think I can sort of ramble on this and you can let me know if I'm in the ballpark.
Of course I am aware the monster math (specifically damage expressions) was a bit borked and needed revision (and it received it). Secondarily, Solo design was initially pretty terrible and really manifested in mid-late Paragon tier when status condition/stun lockdown became a pretty easy power play to pull off. Perhaps the sort of issues that you're speaking of from mid-paragon tier and beyond took place before these revisions?
Beyond that issue, I can speak to a few things that serve to make late Paragon (specifically 17 onward) difficult for GMs to recalibrate their methodology (encounter budgeting and base difficulty):
1) PC Action Economy Growth.
More and more off-turn (immediates and frees) actions and Minor Actions are becoming increasingly available to PCs. The value of these abilities (especially when buffed with force multiplication and upward scaling damage) is extreme.
2) Pervasiveness of Action Denial or Punitive Status Effects.
That pretty much speaks for itself. By level 17, there are 10 available Encounter Attack Power "Stunned UtEoYNT" abilities available to various classes (Druid, Fighter, Paladin, Rogue, Wizard). Blinded, Immoblize, Weakened, Restrained, et al are all also becoming increasingly available as well.
3) Force Multiplication.
Yup. 4e is definitely a team-based tactical affair when it comes to combat. X-Men meets Dungeons and Dragons and the "Fastball Special" keeps getting special...er. By level 16, PCs will have their capstone PP feature and that coupled with various other synergistic effects can make for brutal novas, enemy control, or lockdown combos.
PCs are also much more hardy as they have more access to effects that outright deny damage (Immediate Interrupts and/or Resistance). However, those 3 above have primacy IME.
However, GMs certainly have the tools to respond to these things. It just takes more and better understanding of said toolset (synergy between monster groups and battlefield features, force multiplication of hazards/traps + empowering runes/glyphs, punitive catch-22 auras, features that cancel or lessen major status effects, and great tactics generally) to pull off. There is no doubt that it takes a more skilled GM to manage the tactical overhead of late Paragon Tier 4e versus Early Heroic Tier 4e. Undoubtedly. However, the tools are in play to do so.
That being said, the encounter budgeting of 4e does get stressed by mid/late Paragon Tier. There is no doubt. It is infinitely more robust than ECL and CR (which I found to be solid up to 7, decent through 9ish, wobbly after 10, and totally ineffective by 13). You can usually go from a baseline of n (level) to n + 1 and feel confident in the predictability of outcomes.
By late Epic though, I'm probably putting that n + 1 to n + 3.
I do think I understand what you mean though about "the pervasiveness of default PC power and scaling force-multiplication" possibly being an analogue to "campaign disrupting magical items". I just think its pretty manageable just by using the tools in the system and getting better at your craft. I haven't had much, if any, problems with encounter building predictability until about level 27 or so. At that point, the game gets extremely swingy.