I agree there are many who care more about the brand than the substance (BMW anyone?)...heck I live in Los Angeles. However the people I know who are buying the Equus and in some cases Genesis are the doctors and lawyers and senior executives you are referencing. There are enough practical wealthy people out there that just want a comfortable car and in some cases seek a below the radar image. I do think creating a separate brand would make sense, but it would be very expensive. The concierge service of coming to your home to pick up your car for service is a nice perk.
I drive an E Class now, and have had 4-5 MB's, 2 audi's and infinitis. I thought hard about the Genesis before and couldn't pull the trigger. In the part of LA I live in, you will see Genesis sedans all over the place, and usually a few Equuses (?). People are comfortable with a $40-45k Hyundai now, why is it hard to imagine going up another $10k?