Doctor Who: 8 Theories About Ryan And Graham's Departure In Revolution Of The Daleks
6. They Sacrifice Themselves To Save The Doctor
To date, Ryan and Graham have been with the Doctor for two full seasons, and while character development as a whole has been lacking across the Chibnall era, it's safe to assume that they've grown quite attached to the Doctor in that time.
We even see hints of this in the trailer for Revolution Of The Daleks. Graham says "the Doc would really want us to keep an eye on the planet", showing that his respect for the Doctor is compelling him to protect the Earth while she's gone.
Not that it's anything new for companions to be in awe of the Doctor (in fact, the majority of them are), but it looks like this is the case with Ryan and Graham too, and this in mind, it wouldn't be a massive stretch if they ended up sacrificing themselves to save her, voluntarily ending their lives so that the Doctor's can continue.
Now yes, the whole regeneration thing does make heroic sacrifices pointless, but Chibnall could always invent some regeneration-inhibiting weapon to get around this.
Ryan and Graham can get reckless when strong emotions overcome them (e.g. Ryan running around with a gun shooting aliens in the face in The Ghost Monument, or Graham considering killing Tim Shaw in The Battle Of Ranskoor Av Kolos), so if the Doctor's life was in danger, there's a good chance they'd step up to bail her out.