@romeo SC 2 was booming when it first came out and didnt have a steep player drop like BFGA is experiencing. The coals were burning for a very long time but its a different crowd and we cant really compare them to the 40k gaming community because they play the game for completely different reasons.
IF you got a pretty good product people are gonna stay around and play the game especially when it comes to RTS games.
Content is important yeah but if you have a huge player drop in the first month or 2 in a RTS game its not only content related but a myriad of things and in BFGA case. I think it has more to do with Balance, lack of variety/ options and how grindy the campaign feels because from what i have hard not many have even finished the story mode.
The 40k community is a tough nut to crack because they are really damn flakey. They never seem to be satisfied with anything and usually dont stick around It also doesnt help that most 40kers are pretty damn old and got real life stuff on their plate. I might be going into stereotypes but this is the pattern i noticed with 40k games in particular. I see everyone say FOR THE EMPEROR OR THIS MAKES ME HARDER THAN TERMINATOR ARMOR and HYPE HYPE HYPE at the announcement of a promising 40k title with decent potential and only 1/4 show up to the party and half leave. It may be just an assumption but thats how i feel.
When it comes to Fallout 4. That game was a crap shoot to begin with and this is where the content problem sticks. RPGs or Single player games rely on fresh content more than other games. You finish them you run out of things to do and you stop playing. RTS games staying power is often contributed to the multiplayer there are exceptions though like Total War Warhammer which is people primarly play for the single player content and coop.
I assure you if you removed the campaigns out of SC, DOW or CnC you wont see the player base gradually falling. Because these people just want to duke it out and destroy their friends or opponents in PVP.
Where with Single Player content. When you are done with the campaign there just isnt anything more to do and you hop off the train once the replay value wears off.