I don't know how this applies to expansions. Expansions (and Heavensward definitely is) are typically the size of the base game at the very least, if not more. Or, they add a ridiculous amount of depth, items, and systems to play around with. Or maybe it does all of that. It's not vanity, it adds substantial depth to gameplay. I can't speak on Heavensward yet, but the simple addition of 3 Jobs and a level cap increase is sure to mix things up in a big way.
As for 'giving in' to the practice. People here 'give into' (I hardly think this is an appropriate phrase) the practice because it's not a big deal, and it's commonplace. It's like... part of the territory, if that makes any sense. It's the same concept that playing a F2P MMO almost certainly comes with the caveat of needing to empty your bank account to be any good at it. It's the balance of the MMO genre right now.
F2P have massive-scale Cash Shops with much more than just vanity purchases, and generally have much wider gaps in content updates. Non-quality updates, poorly managed, and paying to win. P2P games have (now) low scale, vanity based Cash Shops, with sub fees, and expansion costs. The payoff is higher quality content, with more rigid and swift updates, and a better managed game (comparative to a F2P, or most other MMO's). And if I'm going to be honest, in the MMO genre right now, your money is way safer, and less stressed in a P2P scenario.
Go talk to people who played Maplestory, and literally spent thousands of dollars in short time frames just to compete at the end-game level. There's literally items that are sold on paypal for hundreds of dollars in that game. Or talk to people who played Dungeon Fighter Online for years, and sunk money into that game just for it to be shut down, losing all of their money and hours in the process. Or go play 1 of many MMO's that have 0 community, 0 updates, and no kind of quality content.