I think, the mechanic is Jahil.
Transmission oil's purpose is to provide lubrication between the moving parts, dissipate heat, etc. It also gets deteriorated with the passage of time, and accordingly, must be changed.
Transmissions do not fail when you change the transmission oil. They may fail if you keep on dragging the old transmission oil for long periods of time, or pour in incompatible transmission oil (like ATF in place of CVTF, MTF in place of ATF, etc.)