You absolutely need to spend money in PoE to buy stash tabs. It's basically mandatory if you play regularly. The difference to most dark patterns is that the spending has a very low cap. Once you've spent $50 or so on stash tabs you are set forever and never need to spend again. So it's not so different from buying a $50 game, just that you get to try it out for free first.
$50 is exaggerating it nowadays. With async trade you could buy a single merchant tab to gain access to trade (stuff sells pretty quick with async trade!), and maybe a currency and scarab tab for the bare minimum convenience. Around $20 and you've got yourself a meaty beast of a game.
