I have tended to flip the question you have asked.
Through out my career and private use, I've continually asked myself ... "Is there a reason why I can't move away from MFD and just use 35mm?" It was a logical business question that a lot of photographers were asking for various reasons (partially convenience, partially aesthetics, and partially consolidation of expenses).
Years ago, I compared my 22 meg Canon against my 22 meg Hasselblad H2D in various controlled shooting conditions. No contest. Other than handling, higher ISO and speed of operation that gives 35mm its purpose, the H won every time. Better holistic results made up of a lot of image factors ... including more faithful colors and color separation, smoother tonal transitions, more organic in-focus to out-of-focus transitions, and the less quantifiable 3D realism that seems a trait of MFD.
The most obvious technical variables between the two were: CCD verses CMOS; Optics (L glass verses HC glass); and perhaps the most important difference: Pixel Pitch: The same resolution achieved in a larger sized sensor.
It is no secret that many simply loved the way the "Big Pixel" MFD backs rendered images.
In my experience, when a MFD back has smaller pixels it gains the resolution advantage over current 35mm offerings (60, 80 and 100 meg MFD). When the resolution is similar to 35mm, (16, 22, 33, 40 meg MFD) the MFD back then uses bigger pixels ... which provides that holistic improvement in image characteristics. In either case, MFD justifies itself only IF those characteristics aid in your form of photographic expression.
I've made those comparisons many times with the same results.
My latest was my 37 meg Leica S(006) verses My 36 meg Sony A7R and 42 meg A7R-II. In this case, similar resolution but the sensor size difference is less than comparing a FF 645 MFD back. The Leica still wins, but may not have by such an obvious margin were it not for the S optic system developed specifically for this camera. Where the S further justifies itself is having taken on more diverse applications ... as mentioned previously, the S also is a dual shutter camera and can sinc to 1/1000 with my Profotos.
Personally, I never say never. Who knows what is possible in a small form camera? The A7R-II is an amazing thing. When 35mm can deliver those image characteristics I look for, I'll gladly jettison the MF stuff.
Marc