Interests diverge in another way too @KeohaneDan (cc @anandMenon1). Art 50 is important for EU legitimacy. EU needs members to be able to Leave (even if it hopes none will) as fact that states can leave shows that EU is voluntary/ non-coercive & that states remain sovereign 1/3
If EU knew that states would only decide whether to Leave at end of negotiations, it would of course have incentive to offer them unattractive terms to discourage them from Leaving. So an art 50 w decision on whether to Leave at the end of process could never work 2/3
For Art 50 to work (ie to make Leaving possible while not encouraging states to threaten to Leave just to see if they might extract concessions), decision to leave must be taken before negotiations on departure & future relations, not after them (do you agree @piris_jc?) 3/3
