It is much simpler than that though. The UK wanted to be treated as a third country. Third countries licenses are not recognized in the EU. It doesn't matter if one did its complete training in non-UK EU countries, only if the license held at the time the UK became a third country was one issued by the UK or not. It is really as simple as that. Leaving the EU did in fact reset every and all things formerly regulated by the EU to a non-EU recognized status, except if regulated differently in the TCA or the WA.
Again, to change that there would need to be an incentive to do that on an EU level. And to be fair, for british airlines there is probably no wish to do that either, as it is in their favor to trap pilots within their regulatory region and therefore being able to drive down conditions even faster without any fear of them escaping easily elsewhere.