> For instance if the UK had remained and the EU had then announced all the opt-outs were voided, that would have been a major change but well within the remit of what was possible in a Remain scenario (without the threat of leaving what stops the EU doing whatever it wants to a member state?).
Those opt ours were enshrined in the treaties. There was no way to rescind them without the consent of the UK. You’re constructing a straw man.
The EU institutions have done many things they weren't allowed to do under the treaties, so that argument just wouldn't have landed. An example of that was human rights law, which was originally never intended to be law and wasn't written tightly enough to be so. The UK and Poland obtained a supposedly water-tight opt out written in plain language, which the ECJ then simply voided. There are other such cases.
Those opt ours were enshrined in the treaties. There was no way to rescind them without the consent of the UK. You’re constructing a straw man.