Part of the background for this entire dispute is that prior to the OSI's founding, "open source" was a generic phrase which was broadly understood to just mean "the source code is available". See many documented cases in https://dieter.plaetinck.be/posts/open-source-undefined-part...

So it's a bit ironic to argue that terms cannot be redefined, when that's already what happened with "open source" and what got us here in the first place. If OSI had chosen a novel term (e.g. "Sourceware" was one option they considered), they would have been able to trademark it and avoid this entire multi-decade-long argument.