WordPress admin area is simplified already for users & customers if you as the developer provide proper roles for them, that’s what WordPress roles and capabilites are for.

Basically it’s the developers fault, if he/she/it provide an unmodified admin role to customer and customer can’t handle it or breaks things.

The above screenshots unfortunately do not reflect real world usage at all with lots of custom post types & custom taxonomies also added to native post types, removed native post types and all that visible based on custom user capabilites etc.

Better brovide a proper role / capabilities editor in core, problem solved.