Thousands of complex shaders were written for mental ray using the C++ SDK. Studios with massive, proprietary shader libraries (e.g., mib_* , dielectric_material , car_paint ) cannot easily port these to Arnold without rewriting. For these studios, Maya 2020 + mental ray is a preservation tool.
mental ray for Maya 2020 supports exporting to .mi (mental ray scene format). This allows you to render on a render farm without Maya. mental ray for maya 2020