In my estimation there are several issues that are leading to the game dying
1: The "Meta" and Competitive Seasons - For most people the way to play this game is the mp battles. However, the mp battles are not balanced at all and every patch seems to have a secret sauce build that is basically the designated big bad bully of the Meta. Some factions are also completely and utterly garbage in comparison to others. This by itself is bad, but combined with the super competitive nature of the mp ever since the devs have implemented the "competitive seasons" aspect, it is just brutally bad for the mp imo. New players enter into mp trying to figure out what they can make work and it isn't welcoming at all, because the game is extremely punishing if you aren't following "the meta" for the current patch. That stops new players from joining and probably causes people who are better at the game to drop it as well (for example as much as I like this game I haven't played in the past couple of days because I'm tired of facing the same pyro acid tyranid fleets).
My suggestion to alleviate this issue is for the devs to have more frequent balance passes (balance is something that is almost never actually reached as far as I can tell in mp games like this), and to have breaks from the hypercomptetitive seasons. New players aren't going to stick around in a game where they are facing someone w/ 100+ hrs facerolling them with the FotM fleet while they try to learn. Having periods where the competitive aspect is toned down will allow players to try out stuff w/o fear of ranking issues, and before S1 the variety of fleets and builds you faced was a lot bigger in my estimation, which lead to a more varied and better mp experience
2: Skills - The skill selection system the way it is now is incredibly punishing for anybody wanting to pick up the game and go into mp. Some factions are borderline useless without access to certain skills in genera/for certain matchups. (traktor for orks, for example) The fact that skills are still locked behind faction level is competely nuts and a horrible decision, just have them all unlocked at this point. Making someone have to grind through a faction to make them useful doubtlessly drives away new players that the game desperately needs
3: No replay system - This is something that has been brought up again and again because it is probably the biggest reason why the games playerbase is shrinking. Without access to a replay system or observer mode the only way for people to create content for the game is to record mp battles as they happen. This isn't ideal, and the fact that there isn't a replay system means that less people are going to make content for the game.
I bought and got into this game because several Youtubers I watched for other games (primarily Total War) were playing i, and I thought it looked fun so I picked it up. I'm sure there are tons of other people driven to the game in a similar manner. And yet now both of the channels I originally got introduced to the game from are no longer making video content, probably because their is no replay system. If there was one, content creators would be able to cast battles other than their own and make better videos showing off strategies and how the game looks because they'd be able to focus on that kind of stuff rather than their own gameplay. More content made of this game = more players