As I see it some way to approximate the challenge to the party is necessary, not for every DM or campaign, but for the overall health of the game. A seasoned DM can and will ignore as desired, as will many games that run in an "environment-first" mode, but a new DM has no idea where to start, and everyone else can at least occasionally use a tool to "get in the ballpark" for whatever difficulty they have in mind. Both modes of play need to be supported, and no one should run into a wandering Vecna at Starbucks unless it's corporate HQ.
If character level and wealth are completely decoupled (i.e. no relationship between the two is assumed) then the only way to do the above is to make sure the wealth they actually have is accounted for after the fact. There are a thousand ways to do it wrong, but the right starting points, in my opinion, are this:
1) Start from (essentially) naked characters. By "essentially" I mean any equipment that is required to function as intended is not included, like a basic sword and armor for the fighter. It is the only baseline that is objectively independent of magic items. Anything else introduces additional assumptions, most critically that the power of the "assumed items" has been assigned correctly. That's always been a dubious proposition, but "naked" is the same forever. Plus, it means one always adds power for magic items (barring cursed ones, I suppose), which is exactly what magic items should do from the perspective of the player and the character.
2) Don't measure wealth, measure what the characters actually bring to the fight. If a character own 1000 potions, the potions they actually use are the ones which makes the fight easier or harder. Many items come into play every fight, but the ones that don't, particularly the quirky ones that help make D&D D&D, only count when they matter. Treat rituals, especially buffing ones, the same way.
3) Keep it simple. Treat most magic items as something like +0, +1/2, or +1 to the effective party level and so on. The DM should know what the "standard load-out" of the party is, and worry about everything else later or not at all.
If wealth and character level are decoupled for determining "CR" or equivalent, then monster power and treasure level can be freely decoupled as well without side effects. The DM can put down random treasure like mad. The DM can have destitute dragons and kobolds with rings of power. The DM can enable PCs to select risk/reward by in-game research. The DM can put players on a magic item treadmill. The DM can do all those things, at different times. In short, the DM can use the math to support the campaign, and not the other way around. That's better for everyone at the table, and for the entire D&D community.
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On a much less opinionated note, I wonder whether we could consider changing what an "acceptable challenge" means in either the CR or 4e XP budget sense. I'm toying around with the idea of treating a challenge as "poison" which kills the party, in which case a natural measure is LD-50, i.e. a challenge which can be expected to destroy the party with a 50/50 chance. That removes another "should" from the game, gives the DM good info on what the party could handle all other things being equal, and generally means character level and CR are the same basic thing (campaign-wrecking monster powers notwithstanding.) The DMG can still give guidelines for lethality of combat, but without the tacit conceit of an "average" encounter. I think [MENTION=19265]Connorsrpg[/MENTION] would find this more palatable in principle.
Still, it might be a terrible idea.
For starters, even if it works for single characters it might not work nicely for a party. For example, is the typical DM most concerned about the probability that a TPK occurs, or the probability that at least one PC dies? If the party is a single creature there is no distinction, but obviously it is quite different for a party of 4. And although the risk of a TPK grows dramatically after even a single death (at least early in a fight) there is huge narrative distance between losing 1 PC and losing all but 1. Even so, I can't help but find it a little compelling. The classic mirror of opposition fight is a coin flip, and I wish CR or equivalent could capture that simple notion.