I have no doubt that Sony will continue making more full-frame mirrorless bodies. They've been cranking out bodies much faster than lenses, and I wouldn't be surprised if there were another body or two released this year, maybe one that actually can record 4K to the memory card.
I liked the a7 when I tried it, and really like the controls. It has a dedicated EV dial on top that works all the time, even when shooting in M mode with auto ISO (unlike the Fuji xt1's EV dial.) I liked the AF and even liked how the manual focus worked automatically enlarging the image when you turned the focus dial.
I shoot mostly with zoom lenses, though, and it seems like the system doesn't have any constant f/2.8 zooms available, and even the dimmer zooms they do sell seem big and heavy and eliminate the size/weight advantages you'd expect in mirrorless. The lack of in-body stabilization and lack of stabilized lenses is another set-back in low light performance.
Just imagine, if you upgraded from micro 4/3 to a full-frame camera just to gain 2 stops through better high-ISO performance, but then you lost a stop or two due to having a slower zoom lens, and lost another stop or two through not having stabilization... you could end up back where you started in real-world low-light capability, at least while you had the zoom lens on it. With a brighter prime it would blow all the smaller formats out of the water in low light, but that's another story.
Haha, I know, I said it before and I'll say it again: the OMD-EM5, with it's 5-axis stabilization, is still one of the best low-light cameras out there including FF cameras. Only D800, A7s, and maybe 2-3 others would beat it.
As for the A7, however, I'm getting it for background blur, not low-light. I realize that it will approximately equal my PM2 with its 2-axis. My PM2 with the 28/2 and 50/1.7, since I can shoot ISO800, beats my friend's D700 with his 28-70/2.8 because he's losing a stop from the lens and about 2 stops from stabilization. As far as high ISO I'd say the D700 doesn't have more than a stop or so on m43s sensors as far as noise. That sensor is a bit dated.
