It is like walking out on a marriage. Close the door and leave everything behind.
Not a bad analogy. And when a marriage ends, even if one party walks out on the other, it still ends with a division of the assets. And of the liabilities.
Even the Anglo-Irish Treaty of 1921, in somewhat different circumstances, agreed that the Free State would take its share of the UK national debt, and war pension liabilities.
One of the problems with the "close the door and leave everything behind" idea is that it closes off the "what will happen about..." hard questions which need to be asked.