Even if they made it Source Available it wouldn't hurt them much, because Minecraft is very easy to pirate and the reason anyone pays for anything at all is because you need an account in Mojang's authentication servers (which people do not want to move off of for various reasons).
Hell, they could even make it Open Source with a clause preventing other companies from using to code to make a profit. It's too big to fail.
> The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
> No Discrimination Against Fields of Endeavor
You cannot restrict selling derivatives of the software (a field of endeaver), and the derived work must be able to be shared under a similar license.
GPL does not add any restrictions counter to that. It allows you to redistribute and sell copies if you want, you just have to respect your user's freedom by also giving them the modified source code.
Hell, they could even make it Open Source with a clause preventing other companies from using to code to make a profit. It's too big to fail.