9.12Why not let mobile call Process API directly?

Sometimes it absolutely can.

This is important.

Do not treat the three layers as mandatory bureaucracy.

If:

one consumer
simple contract
no meaningful consumer-specific adaptation

then:

Experience API

may add little value.

A mature answer is:

“I see System, Process, and Experience APIs as separation-of-concerns patterns, not layers that must exist for every use case.”

That is much better than blindly repeating MuleSoft marketing terminology.