I have had the Sony A7 for over a year's worth of shooting and bought the M-P typ 240 when my M9 needed a new sensor.
The A7 produces very good results with most of my Leica R lenses, and only works consistently well with a few of my Leica M lenses (mostly 40mm and up). No, I don't own WATE or MATE ... they should do better as they are more similar to SLR lenses in optical design. But overall, right from the beginning, I found the A7 to be a clunky camera to work with. I played with an A7II at the shop last week; that hasn't changed.
I had a love/tolerate relationship with the M9. It produced better photos with a larger range of my M-mount lenses than the A7 (there were still a couple it wasn't happy with, the GXR did better with those due to its smaller format sensor) but I never liked the feel of its controls or the sluggish shutter recock and sound. The old four-way controller and thumb dial was awkward too. I nearly sold it a few times.
When it needed a sensor, I took the M-P upgrade option thinking it was economically the best thing to do, even though I wasn't really fond of the notion of having to spend another $4000 at that moment. But I'm very glad I did: the M-P is slick, not sluggish, sounds great, is much more comfortable in my hands, and produces the results I want. There's something about its files that I cannot get out of the A7 files, despite that the A7 has a stop or two more sensitivity.
(I did solve my wide lens problem ... Sold my Color Skopar 21/4 and Ultron 28/2 for an Elmar-M 24/3.8 ASPH. Satori! Also have been adapting my Nikkor 18mm and Elmarit-R 19mm to the M-P. They're too wide for me most of the time, and both do quite well on the M-P sensor. )
I've completely stopped using the A7 now. It's going up for sale.
For a B&W only camera, I would be in love with an M-P MM, but I think I'd love an "M Edition 60" design MM even more. An M digital camera stripped to the barest essentials and focused on B&W photography ... I don't need an LCD for review, I turn that off anyway, I just need to set exposure and make photographs. I wouldn't care if it didn't even make JPEGs. Just a way to set control parameters (date, time, format cards, etc) is enough.
Do it, Leica! ;-)
G