The issue these days is that there isn't currently another RP/Tank clan to compete with (barring a VH reopening).
Think the underlying issue for RP/Tank clans is that it's just an outdated and restricted clan model from an older era of the game. Back in Pre-EoC most of the activity took place below 20 wilderness where having a clan full of powerful mid levels was beneficial. Currently, almost all activity in the wilderness is condensed above 30 wilderness well within reach of maxed mains, so a clan made up of Rune Pures/Tanks is at an inherent disadvantage.
I mean, sure, they can certainly exist and even grow (even without another RP/Tank rival), but they'll only ever be at a disadvantage in the current wilderness.