They would leave when everything they make is destroyed, but yet again, this isn't a creative server, don't build pretty buildings.
People would start hiding better, but they would also be closer to spawn, which means they'd be much more willing to take the trip. Of course nothing would change for the people who decide to hang around 4.5k and never come closer to spawn, but that won't be too many people.
Yes they do bother the server, their lack of contact with other players means that the server becomes very desolate, which is exactly why there's rarely more than 15 people online at a time.
Of course there's a chance, but that chance is incredibly low when you build far from spawn. You also have a chance of losing everything in singleplayer, but the chance of such a computer malfunction is so low that nobody recognizes it as a real possibility. Freebuild servers have the exact same chance, a griefer could come on, the server owner could decide to shutdown the server, etc. My argument was that the possibility of being detected 200k blocks from spawn was lower than the chance of you losing everything on a freebuild server, and I stand by that. You are safer 200k blocks from spawn on this server, than you are on a freebuild server.The difference to a free build server is that there will always be the danger of loosing everything.