I'm pretty sure the documents do not mean 'relaunch' but rather 're-enter foreground' which your app will do every day.
[edit.... Actually, I think the last statement is wrong. I don't keep a transactionObserver alive unless the app detects that the subscription has expired. Only then do I add a transaction observer. ]
But your question has a better answer. When the app detects that the subscription has lapsed it informs the user that it needs to go and check the status of the subscription. It then does a receipt refresh or a restoreCompletedTransactions (which may require a log in by the user) and returns with the results. This method covers the device that never called a restoreCompletedTransactions and it also covers the device that has two or more Apple IDs associated with it.
You can also use the notifications system but that's much more complex.