For compute offload, Google has indeed done that - the Tensor chips have Google's TPUs instead of Qualcomm DSPs.
Both on these TPUs as well as on pre-Tensor hardware that had Qualcomm DSPs, Pixels would not allow apps access to the kernel interfaces. Access would be blocked or mediated via a separate service process ('binderized HAL').
(Some) OEMs have repeatedly opened access to these kernel interfaces in order to trade security for performance.
Both on these TPUs as well as on pre-Tensor hardware that had Qualcomm DSPs, Pixels would not allow apps access to the kernel interfaces. Access would be blocked or mediated via a separate service process ('binderized HAL').
(Some) OEMs have repeatedly opened access to these kernel interfaces in order to trade security for performance.
(I used to work on compute offload at Google).