Well, Alpaca San, opinions will differ of course, but I don't really consider stash space as something that should be "unlockable content." It's more of a basic requirement for an item farming game, that is currently being used as a reason to force people to play Seasons.
Few people will disagree that the game is better now than during the vanilla AH days, but none of that absolves Blizzard of their pointless punishment of Non-Seasonal players by gating the stash space, of all things, behind the Seasonal wall.
I'd prefer to wait for a few legendaries than to have no way at ALL to get the stash space(currently), without playing seasons.
If Blizzard is dead set on achievements for stash space, then they should have just added some requirements in non-season for people to do.
Hopefully enough people are pissed off, yet don't give in to the coercion, so Blizzard relents and adds the extra tabs to Non-Seasonal play.