My opinion from what I've seen over the years is the game itself is trash and people regularly lose interest. Leaders struggle to keep members motivated, so create rivalries to 'hype' their members up. This gives them a reason to show up (because just showing up to PK some noobs for loot isn't as fun as it used to be, right?) but these clan leaders need to then deliver or fear looking 'weak'. (I know this isn't anything new and has been going on forever, we did it back in Epidemic in 08, DP in 09 and NME in 2012 and clans were doing this way before us!)
I, personally, stopped finding wildy fights fun when Jagex removed the hopping timer as everyone could instantly hop. I felt this changed the dynamic of pure fights and made any fight above circa 50 vs 50+ a long drawn out return fight, typically lasting 2-3 hours. When a clan was low on opts, they would pull to a return spot and try to out-survive their opponents until they grew in numbers and then the opposing clan would do the same. It meant that if you were down by 10 or 15 at the start, it didn't really matter as even if your members were shit and dying at a ratio of 20:1, you could just outlast your opponent if you just kept returning. It became a boring tedious cycle of who had the most capes on a cape counter, that happened in both servers every single weekend.
When you've got clans in a rivalry, who are unable to objectively clear each other fairly (or unfairly), they will do all they can to 'outdo' each other and demonstrate to the 'pure world' and their members that they are the clan that is progressing (or at least not regressing). They turn to utilizing alliances with other pure & main clans (and yes, I acknowledge this isn't a new thing and clans have done this forever) but when both clans have the alliances, that then escalates into botting up armies of 'tank accounts.' When both clans in a rivalry do that, it then escalates even further into hiring 'mercenaries' from Venezula, just to get those capes up in the corner. Everything spirals out of control and you end up in the situation we've got now... where nobody cares about the game anymore and the little enjoyment the people who do play stems from this rivalry 'hype.'
Perhaps the long overdue clanning update that Jagex plan to introduce will do something again for clanning but I doubt it. Or, the community will find a magical savior who will open a clan and create a clan that succeeds with a completely different model, than finds alternative ways to motivate members in game than the multiple remakes/rehashed clans we have nowadays... but again, I doubt it.
As a final side note, I also kinda find it funny how the primary community site/hub for the pure community nowadays, also actively encourages mains to post too. If ever there was a symbol that the two clanning communities are entwinned, it's that - even if it doesn't gain traction from the 'top' main clans.