Have you no heard of the Great Repeal Bill, the White Paper for which is due to be published today?
https://www.instituteforgovernment.org.uk/brexit-explained/brexit-explained-great-repeal-bill
This will enshrine all the European laws from the last 40 years into British law, so there will be no sudden black hole in the legislation. Parliament can then decide as and when which bits it repeals or replaces. Presumably, then, this rule will still apply, and if Parliament thinks it is a good thing they won't change it.
Sure, and what if they don't like that part of it...... They can change it at will (I believe)