I was pointing out mutual gain is surely better.
You forget that EU members, rightly or wrongly, believe they benefit greatly from EU membership. Giving a good deal to the UK could quite easily cause other members to re-evalute that belief and seek a similar deal, which would hurt the remaining members. It could lead to the collapse of the EU, with pain to all involved.
Also remember that any new deal must be ratified by ALL the remaining member countries. Therefore, if even one believes that the deal will hurt them, they can stop it. The only way to be sure of a deal is for it to look profitable to all countries in the EU.
So while, viewed simplistically, a deal could be done which is beneficial to both the EU and the UK (compared to no deal), it would also need to be seen as significantly less beneficial than EU membership to discourage others from leaving AND would need to benefit (or at least not harm) each individual member country of the EU... Which all starts to sound a lot more difficult than just "mutual gain".