>>96344895
and why wouldn't they be able to reverse engineer the rubicon?
or get new renegades that turned chaos after the ultima founding?
it only makes sense if we assume heretics are almost exclusively long war veterans and/or marines don't fall to chaos since the era indomitus
and from the meta perspective of gw wanting the ranges to be visibly different and not wanting to bother with making spiky versions of everything, both model-wise and rule-wise

if there is one good corner to add chaos primaris is the red corsairs, the premier non-legion force of non-long war veterans, who more or less desperately lacks a proper gimmick to stand out as a subfaction of chaos
huron can be the one to have acquired the prized technique and tools to have a reliable rubicon in the forces of chaos, and be the first chaos marine to cross it himself, the corsairs can be where the vast majority of new renegades would flock to, corsairs can be the detachment with a rule that allows them to field a number of primaris units, like genestealers field imperial guard units

it's a win win for everyone, it doesn't break gw's modus operandi, it doesn't require extensive releases, it contextualises where chaos primaris typically are, it gives red corsairs a somewhat unique aesthetic of kitbashes with imperial units, it gives huron a new importance in chaos politics, and it gives his detachment something to distinguish itself from the other chaos marine detachments