I'm used to being able to upgrade my inventory pretty consistently just through playing the game. Never played a game where I was being asked to spend real money on inventory space - but then, pretty much every MMO I've played has started out as p2p (my preferred payment model). In WS, I just pick up larger bags as I level, purchase them with in-game currencies like renown, used to be able to get them off challenges, etc. Same in Rift, where they can be crafted relatively easily, and (though it's been a long time since I've to Buy Blade And Soul Gold) WoW. TERA I remember spending gold to unlock inventory rows, which was never really all that onerous a thing. The last few were expensive compared to the first few, but they weren't overly so. I don't remember having inventory issues in GW2 either (their crafting mats bag was glorious). I've certainly never had people tell me to wait years to get larger inventory before. Just seems like NCSoft has made inventory a very difficult thing to obtain through the game.
Having bag unlocks account-wide would definitely lessen the sting. Just seems like a huge chunk of money to be asked to spend on something so simple. I'm not a "whale" by any means - probably why I've preferred p2p games for so long, as $15/month is a lot easier to budget, and I don't have to worry about being asked to drop extra for whatever new stuff. I'm not sure how long it takes to get Hongmoon coins, either. From what I can tell, it's tied to spending gold for them or something, as opposed to just having them drop in the world through gameplay a la WS.
Anyway, it's just a concern of mine, because the costumes are one of the major draws for me, and I know those are going to take a lot of inventory space (can't really justify spending $15/month on a premium membership AND spending money on costumes, so unless NCSoft comes to their senses, the wardrobe system is out of reach for me).
Inventory/storage is one of the first things I look for in any MMO so OP definitely had a good question. It's one of the reasons I bought master pack when I knew I wanted to play the game, and I probably will stay premium as long as I play (might drop out at some later point, but come back later and activate premium again). Just have to be sure of my main (I am 95% sure) and use pouches on her, then on alts just get in game as I can. I, for one, am not going to spend hundreds of RL dollars on storage in my MMO. I might spend some down the road, but would prefer to get it with Cheap BNS Gold.
What I've found in most MMOs I play is - if it's really the game you want to invest your time in then go ahead and pay the modest sub fee. It pays off in the long run and supports the game. When interest wanes, deactivate the sub and move on. If you buy "a la carte" in f2p games I wager you will spend more RL money. I like games that reward their subscribers with monthly points to spend in the shop. Sadly BnS doesn't but it does still offer perks that are worthwhile as long as you are playing and enjoying the game.