Jon's been doing nothing but training and fighting since he joined the Watch. Everyone else is sitting on their asses while he's busting his leveling up, plus he'll be an unkillable zombie soon.
And Olly ain't saving shit. What happens needs to feel like a betrayal. There's only handful of notable characters left at the Wall. Sam, Ed, Olly and Alliser Thorne. Sam will be sent to Oldtown next episode and there's no way he would betray Jon anyway. Ed will be 100% team Jon after what happened at Hardhome. Thorne is an obvious choice but it would hardly be a betrayal and would lack any kind of emotional punch, if anything I think Thorne will be completely in the dark and end up trying to stop it from happening at the last second. Olly is the only one left, Sam even gave him the speech making hard decisions because you know your right.