Soon enough, Sony indeed provided full frame options like the Sony A7R. That was amazing. Now, it seems outdated, mainly by that thunderous shutter and other tech that almost seems decades old at this point. The product cycles now are amazingly fast. With the A7R2, many advancements came. Now the A7R3 has put many DSLRs to shame as Canon and Nikon seem to lag behind, even if only in what seem like minor advantages - like EVF, stabilization, and ability to take any lens. Some photographers do not feel these are beneficial enough or feel that tech isn't really there yet. However, for some, these are everything, and for some, so is size of the camera. And here, this is where the Sony really enters into it's own space.
What I feel is the biggest advantage using Sony is the diminutive size possible in full frame, high megapixel and ability to take nearly any lens, particularly M mount lenses. The above other advantages are also huge for me, and even central. With the Sony, that is a very small and light camera body, and an M mount lens, you can keep the entire package very compact. Putting on a 24-70, a new AF 85, ZA 35 or 50, or even top level MF Zeiss ZE/ZF 25, 35, 55, 85, + will bring us into, or exceed DSLR category in size - despite the small body. Digital sensors really need the light rays projected straight onto them, leading the designs to often be sizable in glass and length to facilitate that. Granted, who cares if you have a specific task where excellence in quality trumps all else. In fact, it still may be smaller than competition that is actually relevant, including digital medium format. But, with a tiny M mount lens, it keeps the camera - lens and body together - very compact. Surprisingly compact, considering the capability.