That's not all the level 80 Summoner questline is about, though. Broadly, it's about why you fight against Primals time and again, and the same question is asked for the members of the anti-Primal squad. Specifically, it's about the nature of the Echo preventing tempering, and whether that means an Echo-blessed is obligated to fight Primals.
So to replace Arenvald and Fordola, they'd have to find at least one other Echo-blessed young (and it has to be young, since Jajasamu talks about how he feels ashamed of hiding behind his egi while younger adventurers are charging into the fray) adventurer in the anti-Primal squad.
Personally I think the change is mostly to update where Arenvald is going to be. Jajasamu points you to Rhalgr's Reach to talk to Arenvald, who can get you into the cells at Ala Mhigo to talk to Fordola. If their locations are changed, that means updating, especially if Fordola is let out from her cell. Having the 80 SMN questline put Fordola back in her cell would feel odd.