> If the user doesn't trust an operating system, why would they use it.

Because in the case of smartphones, there is realistically no other option.

> For example if they don't trust it, they may avoid logging in to their bank on it.

Except when the bank trusts the system that I don't (smartphone with Google Services or equivalent Apple junk installed), and doesn't trust the system that I do (desktop computer or degoogled smartphone), which is a very common scenario.