I don't think it would be possible for them to stay with AWS considering their storage volume usage. As soon as the storage was out everything else has followed as well
It eventually stopped being a differentiating source of value, and trying to out-commodity the CSP’s on storage cost at scale seems like a bad strategy to bring value back to the product. At tremendous effort you make it possible to lower prices by 20% or whatever, in order to keep the same profit on an undifferentiated product. Who cares?
Surely despite their business being storage, Dropbox would be foolish to design and manufacture their own hard disks?