Balesir
Adventurer
Well, firstly, per my previous post, I think it has always been possible to view the "Dungeon Level Encounters"/CR/Encounter Level stuff as guidelines. The "steeper curve" idea I don't really see from experience in either playing 3.x or DMing 4E - although they had different ways of handling it.The key word here is "guidelines". The older editions were forgiving enough in their math (and the style of play was different as well) that if you threw something at the party they in theory couldn't handle they might get lucky and handle it anyway, or just run away. But the curve was much flatter.
3e (and, from what I gather, 4e) made the curve much steeper.
In 3.5 it was certainly true that uber-high level critters like devils and demons could kill low level characters if they wanted to, but the thing was that they had no reason to do so within the game system and world whatsoever. I once saw a devil taunting low level PCs ("playing with its food", as we termed it) because it had no conceivable prospect of getting either experience or any benefit for its own "side" in the planar wars by killing them.
In 4E, you wuld be surprised at how resilient PCs can be to very much higher level encounters. It's more chancy, to be sure, but the PCs themselves are really quite tough.
The thing about "monster type scaling" in 4E doesn't upset this, particularly - but the reason for it is somewhat different. A level 17 standard Hill Giant, a level 13 Elite Hill giant, a level 8 Solo Hill Giant and a level 25 Minion Hill Giant are all worth the same XP in an encounter, might represent the same Hill Giant* and are roughly as troublesome to kill and as damaging to a party of fixed level. The reasons for treating them differently is really not to do with making an easier or harder challenge - it is to do with making encounters fun and interesting. A party of Level 17 characters against a L17 Standard Hill giant will kill an individual giant easily enough, but there is a chance they will miss it, it will take a few (~3) good hits before it drops and it will have a fair chance of hitting them for good damage. The same party against a L25 Minion will kill it as soon as they hit it, but will be missing a lot. Meanwhile it will be hitting them reliably for fairly minor damage. The combination of them whiffing a lot and constantly getting hit will rapidly get tedious - but the average effect in terms of damage taken by PCs and time to kill the monster will be similar. Likewise, the same party against an Elite L13 Hill Giant will kill it in roughly the same number of turns and take roughly equivalent damage - but they will hit with almost every blow and it will miss them a lot (but do lots of damage, many times, when it hits). Again, not as fun a fight as the Standard. So the recommendation is to use roughly same-level enemies - not out of some sort of "fairness" fetish, but because battles will be more interesting that way. If you want to ignore that advice, you can.
*: The question of having the same creature have a variable number of hit points (L25 minion and L17 standard as the same creature) was brought up above. This is simply a question of how one visualises hit points. They have always, as far as I'm concerned, been a "fuzzy" concept anyway, so adding in a "quantum" element of a sort of "uncertainty principle" to them seems non-problematic. You can either pin a monster down to be easier to hit - in which case it has more hit points - or you can pin it down to having fewer hit points - in which case it becomes more difficult to hit. That is the way the 4E world works and, since you can't actually see hit points as I envision the game world, it makes perfect sense that it could be so.