I dont know where the definition medium format starts and ends.
What I believe to see is a larger difference in IQ between full frame and "small" medium format (like Leica S, Fuji, x1d), than between small and larger medium format sensors. I dont know how much is lenses, how much is 16 bit color, and how much is sensor size.
I can see advantages of the mirrorless cameras like the x1d over a Leica S and also plan to get into a small set of the x1d. But there is one main reason: I like to carry my gear on many places during travel and activity and therefore weight is an issue.
If I was a working pro I would probably much prefer the S system over a x1d, for its fast AF, fast and excellent lenses, fast pace of shooting etc.
Also I think it is very cool to be able to have FF and MF with interchangable lenses and comparable user interface.
So I still find it a very appealing system. I am sure Leica will follow the demand of higher MP in both the Sl and S system. Even though I am not sure we really do need this. I wouldnt mind 36MP in the SL, but I dont really need more in the S. I understand different people have different needs though.
I also believe it is good that different brands go different ways and philosophy regarding their products. This mean WE can choose what we want.
Do we want more compact lenses and body like the x1d, or do we want max. speed and fast lenses with max. IQ (S), or do we want flexible backs with high resolution and even larger sensor (phase and Hassy) or do we want a system where we can adapt all kind of lenses (Fuji).