The mechanics could work well until even later, really. We've got Vlad the Impaler until about 1476 and Cesare Borgia until 1507, both classic CK2 player material. The list of the Scottish dead at the Battle of Flodden in 1513 might as well be a roll of their peerage, such were the number of earls and lords. Babur and his descendants rose to power and ruled much as their ancestor, Tamerlane, did. And, of course, need I say more of Henry VIII and Ivan the Terrible than their names? Really, you could keep this going pretty much up until people started wearing ridiculous wigs. At the very least, I'd take 1492 for an end date over 1453 any day.
Much of what we consider to be the end of the medieval period exists as such because of events that occurred throughout the fifteenth century. Had Charles the Bold and the French magnates faced an opponent less capable than Louis XI, France would likely have continued to be ruled by feudal potentates for some time to come. Richard III was the last king of England to die in battle in 1485, but I doubt anyone at the time would have guaranteed you that would be the case. If we establish a point of divergence prior to such events, I don't think it unreasonable to suggest a fairly long game could exist within the bounds of historical likelihood. I don't consider the overlap with EU4 to be an issue simply because they're such radically different games: EU4 is a game of impersonal global empires, trade, and colonialism, while CK2 is a tapestry of feudal conflict, dynastic intrigue, personal advancement, and finding elaborate ways to marry your siblings.
