Now imagine that the ruleset takes that baseline to zero. You can play the game without magic and it will work. It WILL work!
BUT if you want heaps of magical items in your game it will also work. In other words, wherever on the magical item spectrum you sit for a particular campaign, it will work!
THAT is a brilliant design goal and challenge!
I'm in complete agreement. To my mind the best thing about decoupling magic items from assumed wealth and character advancement is it enables essentially all styles of play (and manner of story) to work better than they did before. In fact, it enables multiple styles of play within the same campaign if the need or desire arises. My other thoughts on the matter are in an older post.
(I would XP you but I need to spread it around first.)
I think most arguments about the magic item economy arise from conflict over what the market for these items should logically look like starting from the assumption that the items 1) exist and 2) the PCs have access to them. I think those are necessary and non-controversial assumptions in D&D, but they also aren't sufficient to tell us much about the economy for magic items. There have been attempts to make guidelines in various settings (like a gp limit for towns of various sizes) but they don't work terribly well with wealth by level mechanical assumptions. It's not hard to see why: the personal wealth of PCs is coupled to their personal power quite directly, but geographic guidance is usually an attempt at verisimilitude within the setting rather than making sense in terms of the existing wealth by level guidelines. Getting rid of wealth by level means we can set economies (or ignore them) in a way which serves the setting and story, without adverse mechanical implications.
In this case I think a useful idea to apply is that of liquidity. The economic rather than magical decanter sort, naturally.

Furthermore, liquidity is not the same as wealth. The king might be worth a million gold pieces, but if 95% of his wealth is wrapped up in his palace he still might not be able to afford a holy avenger, except theoretically. (Like a modern company, a more useful measure of financial health is often cash flow rather than market capitalization.) A setting of relatively low wealth (with respect to the "value" of magic items) but high liquidity within that wealth might have cheap and freely available minor magic items, perhaps comparable to Eberron. The high wealth and high liquidity setting is monty haul. The low wealth and low liquidity game is Dark Sun. And the high wealth but low liquidity setting might be Forgotten Realms.
4e introduces some serious schizophrenia into the equations with the sell for 20% rule to encourage players to hold onto their stuff. I'm no economist, but that implies a pretty illiquid market for magic items. Nonetheless, the number of such items a character would go through over a campaign to meet wealth-by-level guidelines suggests to me the opposite. The common/uncommon/rare system is a salve, but I don't think it addresses the fundamental problem.
I guess my point is I think the notion of liquidity vs. wealth is a helpful perspective when considering the economy of a setting. Decoupling wealth from character advancement could let it become a useful tool as well. I have some ideas about that, but I need to ponder them some more.
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