I wouldn't be surprised if Microsoft viewed Github like McDonalds views the Ronald McDonald House - a long term PR investment that's well worth the marginal cost.
Strawman: Microsoft doesn't require you to come from their hardware for any of their properties, and of course Zune is a dead platform.
Mojang users required to move to Microsoft (formerly Live) accounts: 2021 (https://www.theverge.com/2020/10/22/21527647/minecraft-micro... )
I expect the same will happen for github eventually. Given the were acquired in 2018, by this timeline, I'm guessing by 2025.
That all said I didn't think it was likely that Minecraft would switch over to using Live accounts since Minecraft is so famously cross platform and Live accounts have struggled with that in the past.