But i think you are right. In a Vacuum the EU would probably be quite happy to just restore the previous agreements. From a utilitarian perspective it might even be worth it to sweeten the deal. But emotionally there would definitely be opposition to it and it just sets too bad a precedent.
But there are also a lot of benefits to the EU on the Table here. So if the UK genuinely and sincerely wanted to reenter permanently. I am sure Brussels would find a way to get it done.