> What is the principle that determines that this treaty is worth signing but also determines that staying in the EU is not worth it?
That question cannot be answered to your satisfaction without answering this:
> I guess I don't see the sense in which Britain did not already have its sovereignty and now gains it by this move.
The easy way to distinguish the level of sovereignty is not to compare UK/EU treaties before and after Brexit, but to compare them with the treaties and level of sovereignty between the EU and Canada or Japan. The treaty we’re discussing now is of that ilk, and clearly, the level of sovereignty of those countries is of a different level to that of one in the EU. Can the EU make a law that Japan must implement domestically? No, they are covered by international law as a sovereign state. Same goes for Canada, and now the UK.
The argument that the UK had full sovereignty in the EU was a popular argument by those opposed to leaving but it was obviously false, which is surely one of the reasons their arguments were unpersuasive. From 22:31 of Vernon Bognador's talk[0] at Yale (which is fascinating from start to finish, by the way):
> When Harold Macmillan sought to join, he encountered fundamental difficulties, difficulties which probably have never been resolved, and the essence of the problem was one of fitting Britain into a continental system whose assumptions about constitutions, politics and economics was so different from those held in Britain.
> The most fundamental problem, perhaps, was that of fitting a constitutional system based on the sovereignty of parliament and the absence of a codified constitution into one based on a written constitution, the separation of powers, and subordination of the legal systems of the member states to superior system of European law.
Vernon Bognador is British Research Professor at the Institute for Contemporary British History at King's College London and Professor of Politics at the New College of the Humanities. He is also Emeritus Professor of Politics and Government at the University of Oxford and Emeritus Fellow of Brasenose College, Oxford, and has written several books on the British constitution and Brexit. He voted to remain in the EU[1].
[0] https://www.youtube.com/watch?v=WMV9wSSLcCY
[1] https://www.telegraph.co.uk/politics/2019/10/10/second-refer...
Edit: formatting, will I ever get used to this flavour of markdown? </lament>