Interesting debate. I'm not sure I see a clear argument either way so prefer to await the outcome of a BA test case if we ever see one.

I imagine the reason a test case has never happened is either i) the amount of revenue at stake is small and/or ii) BA's legal opinion says the chances of winning such a case are uncertain.

Either way I don't see the likelihood of air travel getting cheaper, so for me it still boils down to a personal decision on whether the saving of an ex EU start and a dropped final leg outweighs the hassle of pointless positioning flights. Or whether it is easier just to go on another airline. The legal considerations don't really come into it.