If you feel the existing smaller realms are "unhealthy" then you may as well have them all destroyed in one way or another.
There's no need to be snide. It's pretty obvious that just destroying realms under 15 nobles would do more harm than good for the game.
Instead, what we do is provide less emphatic pushes to be larger. Such as allowing small realms to merge (and thus become larger), and preventing small realms from splitting (and thus becoming even smaller).