However, people will still have to buy it since the leveling system requires you to go through that expansion.

That's one of the upsides to modern WoW that they managed to solve, with the Chromie Time essentially allowing you to pick your favorite expansion and just play it to get up to current content. It only works because of how separate each expansion is from one another (and how little the plot matters to them), but it makes playing the game for the first time or even on an alt so much nicer.