But the criticism isn't strictly "this game isn't selling well!" In fact, that's not even criticism of the game. It's not relevant to the end user unless the player base completely vanishes, which is not going to happen.
There's something about this game that has people fuming nine months after launch, after 6 more characters have been added, after some single player content has been added, after rage-quitting has been reduced, after matchmaking has been improved, after it's been available for half price or less on multiple occasions.
What is energizing this obsessive negativity? When I don't like a game, I might say so like once and then I move the fuck on with my life. I don't participate in threads for games I don't like or have no interest in. Why would I?
I'm sure the DLC plan contributes to it. Many gamers have an obsessive completionist mindset and the idea of picking and choosing certain content or being forced to lay out a ton of cash to get EVERYTHING must chafe. Being able to earn some stuff for free wouldn't be sufficient comfort for them.
The fact that's it's not on Xbox probably draws extra ire, as Rise of the Tomb Raider did. (Not saying they're equivalent business deals, just equivalent outcomes.)
The most charitable explanation I can come up with is that most of the complainers really love SF and so their disappointment continues to smolder. I don't know if I believe that one, though.