Although I have no problem with granting all pre brexit atpl holders a blanket exemption to be issued with an easa atpl. The problem is that should easa give uk caa atpl holders, easa atpls going forward. The uk caa could and would be able to relax standards. At the end of the day the uk isn't part of easa anymore and anything could happen with regards to atpl issuing requirements. In my opinion it would be reckless for easa to give such a blanket exemption without first entering some sort of negotiations (witch in all fairness will happen at some point in the coming years).
Then there is of course the point of fairness, as has been mentioned previously. There are hoards of other pilots world wide, with third country licenses who would undoubtedly also like to have the opportunity to be issued with an easa atpl. How fair would it be to only grant this to uk licence holders and not of say australia or argentina.