The prices won't really change if this happens. Just as very few Germans purchase tickets which may be flown out of order because the prices are so high, the number of people who will engage in hidden city ticketing fraud to take advantage of xEU pricing will drop if the practice is mandated but is expensive.

Don't forget that there is nothing wrong with so-called xEU ticketing. It is dropping the last segment of the positioning flight which is causing the angst. The only reason someone based in London would fly to DUB to board a flight back to DUB so that he could get to JFK is to save money. If dropping the last segment is prohibitively expensive, either people who want xEU tickets will be flying a lot of extra segments out to DUB and then back o n a positioning ticket or they won't be saving anything.