I think the reason wins are slumping is due to the grass is greener effect. A lot of inexperienced/incompetent/non-team players see chaos doing well and then try to jump on board to find the grass not in fact being greener. A lot of times in those losing games, I'll check the score board, and see half the team without any significant amount of points/objs/kills/revives/assists. One of the biggest things I see in regards to chaos winning: whether or not people have your back, and that just doesn't happen in those losing games for one reason or another. When we win, it almost feels like a premade rather than a pug.
I think Chaos shouldn't be comparable to Space Marines, at least in terms of very special equipment, because if you think about it from a lore perspective it is accurate. Space Marines always operate in squads and always move together, effectively meanwhile Chaos tends to operate in a less than formal standard, flinging themselves about at whatever they want to kill. This is accurate for the game as well, at least from what I have seen. Space Marines tend to have more guilds and more players, and Chaos tends to have less team play and less guilds, everyone doing whatever they want whenever they want, which is accurate. I think Space Marines should be rewarded just as they are now for tactical game play, meanwhile the Chaos faction should be big risk-big reward style game play. Chaos players should be able to throw their weight around and feel the impact of their accomplishments, reveling in their own superiority. They should have the most offensive capabilities and least amount of team oriented game-play, as all Chaos Space Marines care nothing for their brothers, only for themselves. While Space Marine players should feel strong cohesiveness to their squad, and have clear cut goals and objectives. Like better mounted weapons and defense items, like the storm shield for example.
The World Eaters may conduct themselves in a less than orderly/tactical manner, but legions like the Iron Warriors and the Alpha Legion are no slouches in organization and planning/teamwork. They will add specific flavors to each of the subfactions as the game develops though, so your idea can always be reflected through subfaction traits for certain legions once we come to that point in development.
In reality, CSM tends to be cooperative and teamwork driven or"Undivided" as Space Marine players throw their weight around and feel the impact of their accomplishments, reveling in their own superiority, least amount of team oriented game-play, care nothing for their brothers, only for themselves but that is far from true as CSM also varies depending on the day. Alpha Legion here. I actually made my loadout to specialize in different things but does not fall from needed roles like Sorc or Anti Tank while steering off the needed objective from my guild peers to take other points or decapping more than anyone else Basically making sure they win while doing their job. https://streamable.com/4ln3
In real life Chaos players had great teamwork even among pub groups, for the most part. There certainly was a degree of logging on at the wrong time and getting into games with guys that don't seem to know what they're doing, but more often you'd have guys working together and working on vehicle control. Last weeks however it seemed like more guys that weren't doing what they were supposed to, and maybe a few salty vets lamenting about it. But no, it's been a recent trend. Chaos was a strong playerbase at one point, it was better after the content patch, but I've been out of town so I don't know about currently.
So a recent observation is that Chaos fields an astronomical amount of Sorcs.. like... Holy shit.... and very very few Tacticals or GAs capable of capping points. I noticed that by switching to my GA from Sorc (I used to always play Sorc because no one did) a stalemate turned into a steamroll because.. well.. someone was capping points. This isn't true in all games but it amazed me to see.