The problem of items being subbed in at level persists in regards to looted stuff. If a character kills another and takes their magic sword, they shouldn't have to wait until they level to use it, furthermore what they "spend" on it at level shouldn't influence its power. I'm not arguing that concrete wealth is the only system that works, though I haven't tested any of my usual alternatives in D&D as I don't play it. I'm simply saying that tying wealth to the level system inhibits fluid growth.

D&D makes alternate wealth systems interesting simply because it treats wealth differently than most other games. It is tied very heavily into power, every character is expected to have similar amounts of it, and wealth is expected to grow at an astronomic rate. Getting a flawless system out of it is difficult, particularly in regards to expendables, as those can normally be given downsides to carry around (Yeah, take as many explosive vials as you want. Just understand that it increases both the risk of being blown up and how blown up you get.), where D&D has only the encumbrance system.

With all this focus on D&D, an outline of a functional wealth system that doesn't require giving all goods to some leader, for comparison purposes, is helpful. As such, a quick sketch of one is included in the spoiler block below.
One can classify general wealth easily, particularly with classes. Impoverished, Struggling, Comfortable, Rich, Very Rich is a decent generic scale, but a fantasy one might be Serf, Civilian, Merchant, Noble, Royal. Obviously there is overlap, petty nobility might only manage the merchant level, very rich merchants might manage noble, and there are edge cases where royalty might be down to a civilian level.

Then, goods are defined with the level. A single sword might be Civilian level, where it is a major purpose for a civilian, a shop might be that for a merchant, a contingent of guards that for a noble. Getting goods well above your level would let you increase it, for instance if a homeless guy (Serf) found a bag of gems (Merchant) they might be able to buy themselves up to Civilian level. You can purchase one good of your level before being exhausted, at that point you are counted as if you were one level lower until you manage to get income. The serf who blows their gem bag on a shop and initial merchandise lives as a civilian until their money comes in.

There are a few ways to simulate that. For adventuring types, resources will float around, they might travel up and down the scale a bit. They may be able to buy a noble level good, in their case probably a magic sword they send people to find (which may only be merchant level if you actually go grab it yourself), but in general they lack a permanent level. Otherwise, a profession skill rolled against a target every applicable time increment with a bonus that increases with each failure works. A simple system might have 1d6+skill+1 per failure vs. 10, with a 1 week period, which is potentially useful for city campaigns, others would be calibrated differently.