There's no reason for them not to. Open source launchers using the "honor system" for account verification are already established and normalized. It's trivial to just comment out that verification. The jars and assets are free to download from Microsoft's servers without needing an account. It's a trivial game to get without paying, so I don't see any downside for them to open source the engine.
Honestly, I would almost settle for Microsoft open sourcing the Minecraft Java back-end server at a minimum. This alone is long overdue. The massive fanbase could have started to maintain it in ways Microsoft could only fathom.
the client and server are the jar file, but net.minecraft.client deleted, client has both.
[dead]