Why would anyone expect Google to attest to the safety of a rooted phone for financial transactions where it is directly in the payment chain? No one in the payment chain would allow that.

They wouldn't, that is exactly what I am saying. A phone isn't merely a standalone "computing tool" but represents an ongoing relationship between many corporate parties. Reasoning about the phone that way, as something like a PC from the 80s being encroached on, is an error. It is derived from entwined corporate interest from the beginning. The only similarity is the payment structure, lump sum or finance to "own".