With the caveat that it's been actual years since I played it, the F2P model was purely for cosmetic items and more inventory space. There was absolutely nothing to do with progression tied to it.
The reasoning for the ridiculous grind was that they wanted the game to feel more like Diablo 2 than Diablo 3 did, essentially. But it was ludicrous. It was possible to make your character essentially non-functioning by picking certain weird talents/abilities, but with no simple way to revert it. There's this huge skill tree, but no way to do a mass reset, so you only see a very small part of it unless you go full poopsock.
I'd like to play more of it, but I don't want to commit the time.