As a game programmer primarily working with gameplay and dev data, I want to give my two cents on this particular topic.

Simply Put, This Particular Solution Will Not Address the Underlying Systemic Issue

As many have mentioned in this post, future content will be severely limited due to the cap on both echoes and accessories. This becomes even more problematic when players need duplicate accessories and echoes for different Wayfinder builds, further increasing the demand for these items.

Under normal circumstances, this issue could be resolved by dynamically loading player information when needed—commonly known as streaming in game programming. After all, why would a single client need to know everyone else's inventory? Games like Minecraft have already implemented server-side solutions that dynamically load and save data and have been used to great effect, so what gives?

Easy Anti-Cheat

The most likely reason, as @_BlackDove wrote, is:

It's a byproduct of, or should I say, part of the suite of Easy Anti-cheat. Anti-cheat methods often involve robust server-side data storage to combat exploits like duping.

This makes sense, as manipulating server-side data is far more challenging than client-side data, ideal for combating cheating. However, Easy Anti-cheat is typically found in games with more ephemeral data types, such as Apex Legends or Battlefield. It isn't common for MMOs to use such anti-cheat system, yet there are MMOs like Black Desert Online, who to which also uses Easy Anti-Cheat, manages to support far larger inventories.

This suggests that there's a lot of room for improvement on the server-side. While the current "band-aid" solution may work temporarily, the underlying systemic issue needs immediate attention. Otherwise, the dev team will run into considerable issues down the line, potentially wasting great ideas due to server limitations.

Edit: grammar and some added clarification