You say that, but some of the games that people point at as having harmful monetization have basically the same system of only selling cosmetic items that don't give you an advantage (e.g. Fortnite). And PoE's stash tabs straddle that line a bit.
As far as I can tell the main difference is a younger demographic and being more pop culture adjacent. I don't think that should affect whether you consider it "bad" monetization, but I will concede that context is important with these sort of things.
Sure, but Fortnite's UI is set up to sell to you the whole time. After every match you can see unlocks. Before every match you spend time surrounded by other players with their skins and emotes.
The relationship between EXP and levels in each season only really reward you if you have a battle pass - so the more you play the more the pass seems like an attractive purchase. Once you have the pass, you're then encouraged to play more than you might otherwise want to in order to max out the time-limited rewards.
They don't straddle any lines, they're well beyond it. Stash tabs are de facto required for trade. One can, technically, play without trade and complete all the game content but the ones actually capable of that are putting in tons of hours and they will definitely use extra stash space.
There isn't really a target player that plays POE for free aside from those that are just trying it out. That's all fine with me but if you're going to get into the game, you're going to pay money.
Some of Fortnite's skins have given players an advantage. They've sometimes changed skins later because of it. Fortnite has other dark patterns and harmful monetization practices too though, from their season passes to their in-game currency.
As far as I can tell the main difference is a younger demographic and being more pop culture adjacent. I don't think that should affect whether you consider it "bad" monetization, but I will concede that context is important with these sort of things.