In the same way however nebula could decide to switch to proprietary licenses and discontinue their open source offerings.
Sure but you’d still have whatever the last commit was to nebula under the MIT license. It can be forked etc etc.
I am sure headscale is great, but its a side project and if so inclined (not saying they are, tailscale seem quite generous), they could kill it a lot faster than Defined Networking could kill nebula. But its all a gamble.