Its in the Feel No Pain rule.   At this point we have 2 sources for Damage Motivation Rolls, Shrouded and Feel no Pain.   Both can be made as an additional save against wounds, but FnP has an exception written in their rules that the save doesnt work against Instant Death.    

A single person could rent a multi-person car, obviously if you have more folks in there, spreading it becomes cheaper. A car can hold a maximum of twelve people.   It's 20 Thrones all the way up for a car, or you could pay 5 and ride the wire.   Hey it's affordable, and if you fall off, no-one will know...

With how little the last 2 editions have lasted  2 editions is not very long no.  And a primarch is not a centre piece model for an army. Every example you have given can be written in to a story and then killed of as needed, none of them alter the core state or type of their factions.  Ork warbosses come and go, hive tyrants dont even get names, C'tan were once mighty otherworldly entities but any and each of them could be done away with or come and go and it would not mean huge cosmic shifts in the setting or faction ( Ctan less so once by now they are infinite poke balls they mean less then nothing).   Primarchs were a core part of the Imperial Mythos, a broken run down and crumbling Imperium. They were myths because they could fix things, they are all of them keystones in the lore of the best selling range ( marines) and the best selling faction ( Imperial). Now chaos got the ultimate cheat code in the deamon primarch. But they were more or less always active just not interested so much in the long war they had ascended and them coming down to burn a sector or 3 was still a big big deal.    Loyalist primarchs have no cheat codes, GW will never kill them, once they come back we are stuck with them, only suddenly the foundations ( rocky as they are post primaris) of the imperium are creaking. Tech too old/not understood, they CAN FIX IT or make new better tech. Imperium badly organised they CAN FIX IT, oh it might take a while but inch by inch the decline has already become a tiny tiny progress. Guard run badly they will reform it, navy keeps re taking haunted chaos ships that keep going by to chaos they will ban it. The Mechanicus doesn't play ball they will help and shield the elements that does. Even the church suddenly starts playing nice. The more of them they are the faster they will do it. The onlt avenue become Imperial civil wars AND THATS WHAT 30K IS FOR.    Now there is a gulf between mostly tabletop gamers and the more lore leaning people ( no criticism to either). And while the former will always win out as mini sales make the $$$.  Making 40k into 30k doesn't make 40k better, it makes both worse. Primarchs in 40k will just lead to more and more of the no consequence start war like fights that already are plaguing the setting.    It will lead to an even greater focus on named character story lines and an even greater sidelining of Xenos.   40k has so much to space to explore and expand without resorting to just bringing all of the mythos into the light.    Give me another Badad war with new characters, new chapters, new stories, over The Lion is back and he is more paranoid then eva! any day of the week.