Ter Stegen will leave if he is made the backup or reserve, it is that simple.
If anything, all these fluff articles coming out to protect him are indicating that he is under attack internally, and thus he needs this kind of support from his journalist friends.
The more I see the media bend over to protect him the more convinced I am he really is being kicked out. This is a quote from Ter Stegen about the speculation :
“If I remember correctly, and if no news has happened in the last few hours, we only have two goalkeepers for next season, Inaki [Pena] and me,” Ter Stegen added, in turn ignoring Szczesny, who is expected to renew his contract."
This sounds like he is trying to speak it into existence and delude himself. As if he is the one who decides who the goalkeepers are for next season? Does he have that kind of information? Obviously not. This is him attempting to comfort himself and wish this into reality.