I would have thought that if Bose-X and Co achieve the goal of a reasonable European IR then the population of 'small' N-reg aircraft will naturally decline pretty quickly. If EASA go for a lighter touch on airframe regulation and recognise US STCs that will further accelerate the move back to European registration.
On the other hand, where we are now, any move to kick N-reg out will be bitterly resisted probably with a positive alliance between N-Reg IR operators and the business aircraft guys who have much bigger issues that keep them N-reg (or other offshore).