I agree that it is annoying having to repurchase the game and expansions on every platform. I ended up buying the base game and multiple expansions on PS3, PS4, Windows and macOS.

As a contrasting example, in FFXI you only needed to buy the expansion once and you would get it on all platforms. Moreover I also think you could download the Windows game client as part of a free trial and keep playing once you subscribed, without having to purchase the base game.