I honestly wonder why the ultra-luxurious still have no diesel.

I think it is technically possible to make a big diesel as silent as a petrol one. The even bigger torque could allow a more silent ride because it would allow to drive with less downshifting. And given the lower consumption it will have a much better autonomy, so all this make a diesel perfect for that kinf of cars.

I heared about a project of V12 cdi for the Maybach some times ago...But nothing followed.