I wonder if Capaldi is unsure if he'll be leaving seeing as, seemingly, S10 will be Moffat's last, and no-one yet knows how he or whoever takes over from him wants to leave the show.
Like, maybe the next person wants an established Doctor and a new companion, maybe they want an established companion and a new Doctor, or maybe they want a Moffat style clean slate. I don't feel like it's driven by some great Tumblr-based conspiracy to get a pretty boy in again.
Yeah, that makes sense. Combined with the problems with filming and scheduling, things are just up in the air.
I can't see him leaving after 3 though, it just wouldn't feel right.