I'll ask you the same question differently. If 50 players (top players I'm assuming you talking about) own 80% of the world but are in different tribes wouldn't you want to them to fight it out to see who is actually the better player? Now if these players were in the same tribe that's a completely different question but if they merged just to win can you really call the #1 player the better player?
Let's be honest, the best player isn't the one who would win. The #1 player would be the only one who hasn't quit yet at the end of the game. The problem here is the end game, not the merging. Look at Yatogami's world that he's so proud of winning. It has 96 players in his tribe, and 94 players in all of the other top ten tribes combined. First off, if his enemies all merged into one tribe, the world would still be extremely unbalanced in his tribes' favor, but realistically, if ALL of those players are active, that's 190 players left playing a world that started with 49,998 players at one point. This game isn't a test of skill, it's a test of endurance. If you literally just don't quit, by the end of a world you're in the top 100 because that's all the active players that are left.
It isn't fun to play the game like that. Your travel times to reach an enemy become ridiculous and just going through 400+ villages and queuing up your troops can take over an hour. Then 9 times out of 10 you're only hitting an inactive anyway because everyone has quit already. So you're putting all this time an effort in and get no satisfaction.
This game has a serious problem, but I feel like this rule is a step in the wrong direction to fix that problem. It sounds like you had a previous world where players were sick of this and chose to deliberately end the world to make it stop. I don't think the right answer here is to demand that they should have fought it out to the end. Those players will probably mostly go on and play on new worlds because the start of a world, and the first few months is SO MUCH FUN. They probably just wanted to be able to experience that part of the game again. There are tribes that start up on EVERY world and do that. They play the first few months, have some fun, and then quit to join on a new world when it comes out. Some people are completionists and just wanted some sense of finality before they move on.
To be completely honest, I don't think the mods have any business stopping them from doing that. It's a game, if that's how they wanted to end it so they could start on new worlds then who are you to say "no, you have to fight it out". If you kept the world going then most probably would have quit anyway.