How do you define mainstream enthusiast/pro?
These would be the people who are now mostly using Nikon, Canon, Hasselblad or Phase?
A high proportion of Hasselblad and Phase cameras are bought by semi-pros or amateurs, but they are a small proportion of the total photo market.
You would be asking what people will be buying then, as most of them will still be using the system they have now.
...and will there still be a market for for high-end specialist pro cameras like Sinar?
The Problem for Sinar is that their kit lasts for ever and does not go out of fashion, so surviving on replacements would be difficult, with so much used kit out there, and fewer people "taking the trouble to do it properly".
The irony is that Sinars and other Medium Format Digital View Cameras (MFDVCs) come in the "mirror-less with interchangeable lenses" category,,, but they are at the other end of the spectrum from the pseudo-SLRs that you would think of, but are they mainstream enthusiast/pro? ...and should you re-phrase the question categories?
With the growing popularity of TS adapters and lenses, I ask myself if there will be mainstream cameras with body movements... as this is so easy without a mirror.