User blog comment:Derax-Pagani/Fairy Tail Chapter 494: The Hill Extending Towards Tomorrow Review/@comment-2214433-20160716200239/@comment-7418318-20160717145328

Maybe it's the other way around.

We know that Acnologia stole the souls of the Dragon Parents, resulting in them needing hosts to survive (as I understood it). Even though Igneel seemed physically and magically on par with Acnologia, he still lost his soul, so one could say that this soul stealing is Acno's greatest weapon... one that he apparently only uses when his regular power isn't sufficient.

Maybe Larcade's role is to prevent Acnologia from stealing souls. Even if he is not powerful enough to steal Acnologia's soul, preventing Acno from doing any stealing of his own would still seal one of his most powerful abilities and thus make him vulnerable to attack by probably August and Irene or by the Dragon Slayers, if they can somehow close the raw-power gap.