most pros dont use crop bodies. while in some cases the difference in the equipment is negligible viewing a image, in others it is all too apparent. Is this even arguable? How many pros use crop bodies. 1 in 500? There clearly must be a reason for this and why the differentiation between a consumer body and a pro body. So yeah, it is based on the image. But the image is reflective of what equipment shot it to a extent.
what are you considering a "pro" level body?
the D600/610 is generally considered a consumer body, as is Canon's 6D.
for a long time, Nikon, on their own web page, designated "pro" level bodies as the bodies that had the built in vertical grips while everything else was "consumer". If you still go by that definition, then the D750/D800 are also "consumer" bodies.
how many pros are now using mirrorless cameras?
really though, if you back up far enough, "professional" photographers doing portraits and weddings werent using the smaller 35mm format anyway. they were using 120/220 medium format. 35mm was a "budget" format.