I think a lot of discussion around Frieren is coming from people who view everything as a commentary on politics and society, when Frieren and it's creators are really just interested in making a derivative commentary on Fantasy tropes.
The show doesn't call them "animal-like", but "beast-like" - it draws a spectrum from "dumb" monsters to those who practice mimicry to the Demons who have internalized human speech. Many of the other monsters in the show are clearly more intelligent than ordinary animals as well. And I've seen many people who comment on the show fail to mention that ordinary animals, too, can have complex social structures and dynamics.
And all that being said, the manga isn't done being written yet, and I think it's clear (and feel free to I-told-you-so me in five years or w/e if this isn't the case) that they're building up to the reveal that Frieren is wrong, the Demons can be reformed.