For the EU, Brexit is all about damage control: protecting its members and the four freedoms, starting with the single market is plain vital. A country outside the EU cannot enjoy the same benefits as its members, period.

As Fintan O'Toole wrote in your quote: One was the simple logic that if a country could leave the EU but still enjoy all the benefits of membership, there would be no EU. Giving Britain what it wanted would be terminal.

If it works for the UK, fine. If it doesn't, tough shit. That the Brexit negotiations have come as an opportunity to rally round common goals and show unity is a bonus, not the main motivation which is preventing the EU from destruction.