Question... In the grim darkness of the 41st millennium, how do the traitor legions deal with their casualty rate?
For starters there are standard warp shenanigens.
-Demonically ascended CSMs usually don't die, but reform in the warp when they are banished from realspace.
-For some CSMs the way time has passed for them is such that although they may be 10,000 years old, they've only really lived about 400 years of equivalent real space time. Which is a lot for a human but about equal to a standard veteran of a first company space marine.
-Standard warp shenanigens that can result in time travel nonsense, e.g. an Imperial Crusade arriving centuries
before it sets out. This kind of stuff definitely happens to Chaos Marines too, and Battlefleet Gothic gives us a canon example where this is done on purpose in the Infinite Fleet. Tzeentch's Infinite Fleet often contains duplicates of the same ship/crew and Imperials often confirm a kill on a ship from the Infinite Fleet only to fight the same ship again in the future, as they're fighting a past version of it.
-Standard warp shenanigens can allow for greatly accelerated recruitment times & rates compared to realspace. The sorcerer planet of Q'Sal has 800,000 years of recorded history in the same time-span of 40k's 40,000 years. In some places of the Warp, time doesn't pass at all.
I mean sure, there were a lot of them that left and joined chaos, but surely that supply of chaos marines must be running dry? The loyalists have the geneseed collection and facilities to create new marines (right?), but the forces of chaos presumably haven't invested in that much infrastructure. And sure, you get new recruits on occasion, but I imagine there are far fewer new corrupted marines than normal folk turning to the dark powers. I have a hard time seeing how that could replenish their losses with how bloodthirsty they tend to be.
Some chaos space marines like the Emperor's Children and the Iron Warriors turn to warp-tech and weird biomancy horrors to mass-replenish their ranks. But there's also plenty of lore where marauding bands of CSMs essentially recruit exactly the same way as fleet based chapters. They show up to a planet, establish recruiting links with local warrior cults/tribes/death worlds/tithe governors and put them through the trials. Suvivors get inducted.
Black Crusade sourcebooks and this one really neat short story, IIRC it's called the long war or something. If I find it I'll post an excerpt of it here. But the tl'dr of the story is a boomer dreadnought who was a veteran of the Horus Heresy invites CSM champions to his fortress for a competition to see who gets to win the prize (a sack of soulstones). The boomer dreadnought is depressed when the CSMs that show up all fall short of his standards:
-The Night Lord champion offers him the location of a weak point in the Imperial Navy's defences, guarding lots of valuable space hulks that can be refitted and made war-ready. The boomer dreadnought berates him for being a pathetic example of the Night Lords, for a true Night Lord wouldn't be asking for his help attacking the Imperium, they would have just done it.
-The Thousand Son Sorcerer pisses him off because he says he doesn't even really know what the big deal is with the Long War, and he only cares about amassing his arcane knowledge and literally doesn't care about the Imperium at all.
-The Emperor's Children champion
seriously pisses him off because he gets distracted and starts bragging about how he stole Typhus's victory from him for fun. The boomernaught gets furious that the champion dared to betray "brother Typhus" who is also a veteran of the long war.
-The Word Bearer champion pisses the boomernaught off the most by saying that they're only here out of respect for the boomernaught's honourable service record. But subtly imply they consider him a has-been, who can't see that the Chaos Legions have already "won" and are free to do whatever they want. The Word Bearer can spread the faith of chaos, the Sorcerer accrue power, the Night Lord be a creepy space goblin and the Emperor's Children do whatever he feels like. At this point the boomernaught asks them how old they all are and their ages vary from 60-300 years old.
The boomernaught is fucking shaken and suffers 40000 stress damage realising how much the galaxy has changed. The Black Crusade rulebooks even bring this up, CSMs are rare and veterans of the long war are even rarer still.
So the answer is... It depends.
Warbands like Iron Warriors, Emperor's Children and Night Lords should be fine. Iron Warriors have geneseed resistant to mutation and shun the warp. Night Lords shun the warp, and Emperor's Children have fabius bile.
Others like the Thousand Sons have more difficulties due to recruiting only from pools or sorcerers and psykers, but with warp fuckery and careful management of what fights they take should stay above attrition.
Word Bearers and Death Guard CSMs actively manage their own worlds with fresh and plentiful supplies of recruits.
Alpha Legion actively steal the geneseed of other Legions, amongst their usual recruitment practices.
I have no idea how World Eaters and Black Legion survive their constant never-ending bouts of team killing and crusading. If the lore was accurate to its own description, I imagine these CSM legions would be diminishing in size.
Rogue warbands like the Red Corsairs basically operate as a standard fleet based chapter.
It's also almost certain that there are just less Chaos Marines now. Around about Horus Heresy the chaos legions inflated their numbers above legion strength and had over 1.2M> marines so there's a good chance they have just lost a god awful number of dudes that they haven't been able to replace