A long lasting clan that you forgot was Revolution of Gaming (www.RevolutionofGaming.com)
However, I feel that clans are something that anyone can make. So they won't stop getting created. Everyone wants to run there own clan, experience leadership and build something out of nothing. Its fun to join a strong existing clan and move up the ranks, I did that in RoG and now I'm one of the leaders and have been for the past 4-5 years. However, you'll always have those people who clan hop and create new clans. Many don't last long though. I know because I run the Xbox360Clans recruitment site.