On every world, there is going to be a group of players who are aggressive and want to fight. They will steamroll this tribe you just described because they do not know how to fight. It doesn't matter how many villages they have; if they suck, they will not survive in the end against players who actually know how to play.
This is a strategy game. No one strategy is any more or less valid than another. Some like to noble barbs; some want to keep them all for farming. Some try to create massive tribes and alliances; others are more elite. Everything we do in the game is done with one goal in mind: to win the game. The mods should not be able to arbitrarily limit our strategic options for the game.
Here are some of the problems I have with this rule:
* It is being implemented mid-world. Tribes set their strategies early on, and implementing this rule change on an in-progress world messes up plans that have been months in the making. It also gives an unfair advantage to tribes that have already merged while disadvantaging those who have had plans of merging but haven't done it yet.
* It is completely arbitrary. There are no definitions are set guidelines. When we ask for clarification, we are told that we have to ask a mod before merging to be sure that we aren't breaking the rule. Any time a human being has discretion to judge a situation, there is ALWAYS the potential for bias. Even when we try to be fair and impartial, we all have biases that we don't even realize. Even if they think they are not doing it, mods will be influenced by their personal relationships with the players in the merging tribes and those in the tribes they are fighting.
* It limits our strategic options for no good reason. In the end, a tribe can only have a certain number of members. Why should it matter how they got them?
* It makes diplomacy a joke. What's the point of fighting side by side with another tribe for the whole game if you're not allowed to merge once enough members have gone inactive in both tribes?
Disclosure: In an effort to be transparent about my own biases, here is a rundown of how this rule will affect the tribes I am currently in:
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Juval: Both sides have planned on merging from the beginning, so I think this will affect both equally. However, unless something big happens, I think it's pretty obvious which side is going to win this world, whether it ends up being just the main tribe or a merge between that tribe and its allies.
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Kronborg: While some smaller tribes may be intending to merge into one of the big three, I doubt that any of the top three tribes wants to merge into any of the others. We just all want to win.
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Leeds Castle: At the start of the world, there was a mass-recruiting tribe, KIS, with three or four sister tribes: KSE and BSA are the two I remember. I think they have all merged into KIS already, so the rule doesn't affect them but it does affect my tribe, OTA, who planned to merge with W/R from the beginning but decided to hold off until later in the game. How is it fair that the tribe that merged early in the game is eligible to win but the other side is not allowed to merge just because they waited?
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Neuschwanstein: There is a big tribe with 200 members that has at least four allied tribes that will probably all want to merge together in the end. I am in a tribe that is enemies with this group, yet I still feel that they should be able to play the way they want to play, be allied with whoever they want to ally with, and merge with whoever they want to merge with. It's not like we need to have more members than them to beat them.