The clan aspect is more suited for people with lots of spare time. It's most fun if you're in one of the top clans, and you work your way up. It's kind of sad the way people will leave those who aren't as good behind as they move up in the world of clans. That's not the point though. What I want to say is that clans are for a younger audience. When you're in middle school, you generally have more free time. Once you hit high school, you may still have the spare time to commit yourself to a clan. However, once you're a sophomore or junior (grade 10 or grade 11) you realize you have your future ahead of you. Some people (like me) attach themselves to friends they've met in real life and find new interests. I know it sounds like something your parents would say, but spending so much time on your computer won't make you happier. If you get out and hang out with actual people in person, it will help your life experience. That's why I think veterans are quitting.
However, it's also not a bad thing. This opens people up to a new generation, possibly a better one for the clan aspect. One quick example I can think of is Peyton Manning of the Denver Broncos. Sure, the Broncos were still able to won the Super Bowl, but that was no thanks for Manning. Their road there would've been easier if they didn't hang on to his legacy. For those who don't follow the NFL, but possibly other eSports, there's FroD of Team Complexity in Counter Strike. He's arguably the best (second best) awper of Counter Strike 1.6, but in CS:GO he is dragging his team down. He's the reason they can't win a game. My final example isn't on a team, but there's Liquid Ken from Super Smash Bros Melee. He was the most dominant smasher in Super Smash Bros' history. Now that he's back, he is nowhere near that form. Everyone else has gotten better around him.
What I'm trying to say is you can't stay on top forever, things change. You might not be as good, and newer players could just be better. Things could come up in your life to take you away from the game you once loved. There's no helping it, but you just have to accept these people are done. Try to embrace the newer players and turn some of the younger, now veterans, into our honored veterans.