And this is why they implemented it as they did. The goal was to entice non-seasonals to play seasons. Seems to be working from what I'm reading.
I don't think anyone can disagree that the rewards for playing season should be much cooler than this. Maybe give some additional rewards for players unlocking the seasonal achievements for 2-3 seasons in a row.
I agree with the fact that the stash tab is the wrong reward for players to play seasons. Stash tabs are things that NS players and HC players need more of. Seasons I believe should get the cosmetic rewards. Not sure about items only obtainable in seasons. If this was the case, I think everyone will be playing seasons sooner or later as if there was an item that was necessary for a particular build, you wouldn't want to miss out on it. I was happy with the previous arrangement where seasonal players get exclusive access to certain items for a season and NS would get access at the end of seasons.
Personally, I think cosmetics are the way to go... isn't game breaking and lets seasonal players show off their accomplishments without breaking the game.
I myself probably wouldn't play just for cosmetics unless they really struck me as cool. They usually don't. Not into banners, wings are good for maybe 90 seconds and then annoying.
Stash tabs to me are a quality of life improvement to be sure, but it IS a tangible improvement.