Clan Wars and MCSG both prove something and are equally important, however the best clans are able to dominate in both places. Clan Wars is the quickest way to gain reputation, despite, other factors in the game, people only care about the score not the reason why you lost. On MCSG, it is up to the opposing Clan to Accept the challenge and in all honesty if they just beat you, it makes no sense to you again even if it is on a different playing field. However, I personally believe people don't value Clan Battles enough, because they are a MCSG Clan on the MCG servers.
League of Legends and Counter Strike are blamed often on Clans failing recently, however, you have other problems to blame it on their poor time management, and it has been proved countless times by Forgotten with Dungeon Realms and League of Legends and more recently with the majority of Rebels playing CS: GO.
I think the reason people believe Clans are dying is because there is no more defined best, Rebels is by far the best Clan out there, not only currently but in the past and on paper. In the latter of half of this year, there has been not been a definite second best in a while. People may argue Rebels and Rivals were #1 and #2, but there are other factors besides Leaderboards like Triton only joined the leaderboards, recently and their rank doesn't represent their set of skills.