Here's a thought. Instead of instantenously closing a continent, why not just prevent any further emigration or new character creation in one of the game worlds? Make it a case of 'Well, Kepler continent has had a good run. We don't want to bring it to a sudden artificial end though. So although the trade ships can no longer make it to Kepler continent due to choppier waters, you can fight it out to the bitter end and give Kepler continent the send-off she deserves!'
Doing it like that makes for a more staggered, natural reduction of nobles over time. You could even bring in these ideas of reducing regions from a decent roleplay perspective. 'Due to being cut off from the known world and the increasing anarchy, as the nobility die out, regions merge and become larger as you desperately try to hold your realm together.' You could do it in increments as the nobility reduced and realms fell. No doubt you would eventually be left with a hardcore of nobles and players who would never give up on the continent under any circumstances. But although the continent would have much bigger regions, perhaps fewer realms, they would still have their continent and that's the important thing.
Maybe if the player base does increase in the future, you can re-open the continent to emigration and new character creation. And if things really look up you could even reverse the changes to region sizes. But if things don't, at least it's still there in some way for the people who really care about it.
