GrapheneOS actually recommends against using Aurora and instead just using the Play Store, so this shouldn't really hurt users. For extra privacy, you can sign into the Play Store with a Google Account that isn't tied to anything else.
The main reason for me to use GrapheneOS would be to sever the umbilical cord to google. I don't really see the point of using GrapheneOS instead of Stock Android if I then have to use the play store.
Also on GrapheneOS, Play Services and Play Store come unprivileged, sandboxed like any other app. So Google is not an admin on your phone, which I would argue is one step towards "severing the umbilical cord".
Moreover, GrapheneOS doesn't have any issue with apps sideloading.
And more. There are many reasons to use GrapheneOS.