I'm really not a lawyer, but I'm skeptical that such a thing is even possible; is it legally possible to say that you as the copyright owner will never relicense something?
(What I'm given to understand does work is using a copyleft license and taking code from multiple parties without a CLA, because then relicensing requires all the copyright owners to agree, which for a large enough project is impractical.)
How about a standard entity "OSI perhaps?!", that commits a file to an early stage of the repository (could this be automated), who then cannot / will not give approval for a relicense?
> I'm really not a lawyer, but I'm skeptical that such a thing is even possible; is it legally possible to say that you as the copyright owner will never relicense something?
It’s possible to say anything. Without something like a contract with reciprocal commitments to make it binding, the legal effect of saying it is limited (though not necessarily zero, because legal concepts like promissory estoppel exist.)