This doesn't strike me as that strong of a monopsony. There are already other well established streaming services; and it isn't that uncommon for users to use multiple services.

If Sony tried doing anything crazy with their monopsony power, studios could just license to Amazon or Netflix (or try out any number of the smaller services). This hypothetical fight would probably hurt both crunchyroll/funimation and the studios; but it would be an existential threat to crunchyroll and funimation.