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They would not even need to open source the servers. Just making the server available for users to run (even under a proprietary license) would be enough.
Or a patch to strip out the online portion. If developers know they’ll need to create that patch eventually, then they can design the game around it. Offline/LAN play/local servers were the norm until ubiquitous high-speed internet.
There’s no technical reason why Diablo 4 needs to be online only. It was a design decision made for DRM and microtransactions. D2 still works great and has thousands of active players.