Tribes with over 40 members fail...
Except the leaders are organising 24h a day
Wrong. I've seen tribes with 120 members that were extremely organized, extremely effective, and extremely successful.
World 2, MPX Family had IDK 600 members? It was the MOST organized tribe EVER. As soon as I joined, I got an automated message in game saying I had joined, gave me a login ID and password which it created for me, for external forums, as well as tools to use in the world which were exclusive to the tribe.
The amount of players doesn't make the tribe. The organization and efficiency does.. IDC if there are 3 or 100 members. It depends on the leaders skill to keep it organized, and relies on the members to step up and be an asset to the tribe.