It is not so hard to understand. Even if the British ATOs remain aligned with EASA regulations, the UK CAA won't be under the jurisdiction of the European Court of Justice and EASA's oversight. British ATO's need certainty and can't wait till the two parties reach an agreement and since it seems we are going straight into a no-deal Brexit, their only option is to apply for an EASA ATO licence.
Are you arguing that the FAA and the USA itself are under the jurisdiction of the European Court of Justice? There can be no assumption that an agreement will not be reached although I understand the concern. Otherwise I believe I made my concerns clear.