I'd guess that there are still loads of people who buy every Battlefield, but only dabble with the multiplayer for a few weeks. The single player will be to add enough value to the package to make sure they keep on buying.
Maybe the disaster that BFV has been with all the bloat convinces them to go MP only, but I'd be surprised.
Single player campaigns for games like Battlefield are over-expensive marketing. Does anyone even like them that much? I haven't even finished all of BF1 or V's because they are both so stunningly mediocre. I feel like they just use the cutscenes for trailers and then make a roughly four hour, underwhelming experience just to tack on.
Just look at the new Call of Duty; yeah, it has historically been known for its campaigns, but then again only a small minority of players even touch it. They even scrapped it for Black Ops 4, not that it was the right call to make given the franchise's history, but nonetheless most friends of mine playing the new CoD haven't even touched it.
Here's what they need to: Invest in Operations. Operations worked brilliantly in BF1 when it fused the multiplayer experience with the actual history of the war, creating an immersive game mode that was supposed to bring you to the battles. Using their incredible score, sound design, and narration, they were able to bring you into an engaging experience based around the Battlefield. It was basically what the campaign tried to do but actually engaging, enjoyable, and very replayable. I was just playing BF1 last night with some old friends and they still love Operations. Grand Operations was a huge misfire that absolutely gutted the potential WWII Operations could have had (and I'm still so angry we may never see Iwo Jima at the very least), but that doesn't mean the mode should be abandoned.
For the next Battlefield, I really think that Operations should have the red carpet rolled out for it like they did for Firestorm, except actually have it come out at launch. Since I assume they will have a return to form in the Modern Era, they could create their own conflict that would normally be explored in the campaign, but instead present it through Operations. Maybe they could even add AI, turn the narrators into characters, and make the opening intro a cutscene that details the progress of the war up to that point and fills you in on the objective of the mission. The team that wins the game could effect the narrative going forward, too in terms of how the next map is presented potentially. There's a lot of potential to be explored here.