My core argument against the unilateral retractability of A50 is that a party acting in bad faith might engage in blackmailing the entire Union in order to get a better bargain.
Let’s assume that the UK, after being unable to find a deal that suits it, decides that it will stay in the A50 waiting room for another two years without approval from the rest of the Union. How? Well, by first retracting their notification and then handing it in again, granting it two more years to negotiate their position. Or, it could be a possibility that the notification is retracted to fight policies that the UK may see as detrimental to itself in the future.
As Theresa May said, Brexit means Brexit. And we in the EU shouldn’t put our common future in peril over sentimentality for a country which already once said it doesn’t wish to belong to this family of nations.