First I want to state some facts:

LWB W221's lenght is 5206mm.
LWB S65 AMG costs 206,500 euros (in Germany).
S600 features 517PS, S65 AMG features 612PS.


Lenght:
Now, in the case MM made a MM 51 (indicating a lenght around 5100mm) the car will be shorter than W221 S-class. That does not make sense. Even if they make MM 53, the car will be still too close to W221.

Price:
The "cheapest" MM 57 costs 365,500 euros (in Germany). That leaves an approx. 160k euro gap between S65 AMG and MM57. OK, that's enough to fill it with a smaller MM.

Power:
MM 57 & 62 feature 550PS, 57S features 612PS. So, there is no room for to squeeze the right engine in while not to interfere with W221 on one side, and MM 57/62 on the other.


So, why would anyone buy a MM with same or even less power than MB W221, similar size than W221, same features than W221, and similar shape - but at addtitional price of approx. - let's say - 50-100k euros?

Has MM brand really enough brand cache to steal customers from W221?

I agree with the others: coupe / cabrio are more possible to appear.