In cases like this it's common sense that rules tbh.
What would Britain gain from a decent Brexit deal? Best of both worlds, non EU trade and good deal with EU.
What would EU gain from a decent Brexit deal? Continued trade... BUT any decent Brexit deal would be a message to EU nations that, gasp, they can work effectively to the same level or better outside the EU.
So which side has it as a core interest to have Brexit fail? Clue; the entity is called EU.