That it’d be some more work is hardly an excuse for a company of Activision’s size, especially for a classic game that was no more than a few gigabytes in size and an update that makes it ten times bigger. The rest of your reasons don’t seem like they should matter at all. Separate downloads that you pay for can also be subject to DRM, and there’s no reason that they shouldn’t be able to control and see the moneyflow from a separate download. It would still be something that is attached to your Blizzard account and game.