There's a lot of bad info on this page, I'll clean up what I can but there are over a dozen missing expansions/adventures and I don't feel like learning the formatting of this wiki's table layout just to add them all. None of them have been "discontinued," that's not an accurate way to describe standard format rotation (and in fact they have done promo offers at various points offering pretty big discounts on packs of old sets, and there's even a special type of pack that can contain cards from all the Wild-only expansions but only the Wild-only ones as well as a similar pack for Standard-legal expos. Standard is one of many modes, it's the most popular constructed format but that's precisely BECAUSE old sets rotate out so you don't have to buy almost a decade worth of content to be competitive as a newcomer; every single card is still playable in Wild, and there's even Classic format if you want to play the oldest cards in the game without any of the balance changes that have been made to address broken combos and whatnot with newer cards. If anything should be a thumbs down regarding HS's content model and monetization it should simply be how expensive it is to keep up, and perhaps the fact that Battlegrounds perks, unlike every other paid feature in the game, are purely temporary bonuses that simply expire when a new expansion comes out and leave you with nothing permanent to show for it.
idk of any successful CCG that hasn't done something akin to a Standard/Wild format split, certainly not one with a dozen and a half full sets plus a few adventure packs for good measure (and nowadays, another completely separate game mode with its own completely separate card pool - Mercenaries). HS deservedly gets criticized for high cost of entry to competitive play, but introducing Standard and Wild formats was the most f2p-friendly thing Blizzard has done aside from not charging for initial access to the game in the first place.