> Microsoft may have a monopoly on Minecraft, but they still have competition from other games (Roblox, Fortnite) and other forms of entertainment (social media, youtube, books, IRL friends).

Which is not at all the same thing, not least because software has a network effect. It's like saying that a company having a monopoly on cars is fine because you can walk or take the bus.

If your claim was actually true then copyright would have no purpose since "granting a monopoly" is its mechanism of operation.

> To me, the problem is more one of the terms of the deal changing; if a person brought minecraft with a 'mojang' account and loses their purchase when those accounts disappear in favour of 'microsoft' accounts, for example.

How does that help you when the next game comes out, requiring a "Microsoft" account from the first day? The problem is inherently that you can't get the game from anyone -- even a used copy from an existing customer -- without subjecting yourself to the requirement you reject, or de facto being stripped of your rights under First Sale.