Because "It'll be alright on the night" my good chap (or "chapess"?). Remember that the likes of GAMA, are essentially, still dictating terms/legislation to the UK CAA, as well as influencing EASA. I'm not convinced that all the UK trained and UK CAA issued EASA licence holders will be left crying into their Corn Flakes, wherever in EASA land they may be flying. Neither am I convinced that Lord Smartypants (CP at BA) and his opposite number at EZ et al Brit airlines, will be expected to be a "Third Country" licence holder. PAH!! (spits on the floor), can you imagine any of these people actually being likened to a (insert, overly dramatic, "Psycho" film musical overtone here) FAA/American Pilot???!!!!! Heaven forbid. They'd have to prove their mettle all over again by sitting a load of nonsensical, boring, laborious, time consuming exams, to prove that their 5000 hours in multi crew jets were actually worth anything (??).
No, no, ...... tranquilo/tranquila......they won't want to be made to sit all the 14 exams (can you imagine an 80's BA 747 Jockey having to do that!!!). Sitting the exams and doing flight skills tests again, so as to be issued an EASA licence off the back of their ICAO issued one, won't happen. HOWEVER......I will absolutely laugh my pants off and shout from the top of some place, very high, if they are made to. Maybe I will see a lot of you at Bristol Groundschool and, they had better get some bigger premises. Theres a couple of big hangers for rent in Cardiff I think? Maybe UK CAA issued EASA pilots will be given a sensible validation option? Maybe EASA, in it's infinite stupidity, sorry, I mean wisdom, will actually, just pull the cat out of the bag and let everyone off? With certainty though, I think I can say that everyone will be "safe" and it will be a storm in a teacup.
Thats said....if they DO come up with a sensible validation process, for ICAO ATP holders and the likes, at least we can bring a load of Americans to GB to help with the pilot shortage. WINNER! WINNER! Chicken dinner!