Hello heretic scum. Can I interest you in supporting the Ecclesiarchy? Benefits of a donation include being made of strong metal instead of puny plastics, no new models for over a decade, half of your old range including your only transport disappearing from sale. Donate now, and we'll throw in no AA at all to shoot down those Emperor-damned Helldrakes.
While Sisters are easily the most hard-done by army, they've also always been a 'fringe army', similar to the likes of Catachans, Inquisition, Harlies, Genesneaker Cults, etc...
Chaos on the other hand is probably the single most important faction to the entire setting, as it is THE antagonist around which the entire setting revolves... It is the ultimate threat to absolutely everyone. The Old Ones were devoured by the Warp Entities spawned during the 'War in Heaven'. The Necrons went into stasis in part because of the birthing of the Gods. The birth of Slaanesh crushed the Eldar empire of old, and she remains their inevitable doom. It was the Chaos Gods who broke the power of the Emperor and the fledgling human Imperium, and now doomed humanity to a slow & inevitable demise. Even the Tyranids are not immune to the mutating effects of daemonic warp-plagues!
Everyone has reason to fear the Chaos Gods and their servants, and yet, this fight is also a futile one, as Chaos can never be destroyed - the inevitable victory can only be delayed.
Chaos Marines are the most recognisable face of that force, and are considered one of the 'Big 4' armies alongside Loyalist Marines, Orks & Eldar.
Both armies are actually to the point of needing full-on 100% overhauls, but guess what? I'm sorry to say that CSM's are simply far more important than Sisters of Bitter, and always will be.
40k can exist fine if Sisters go the way of Squats.
The setting itself would need a huge re-boot/overhaul if Chaos Marines were to suddenly disappear!