Or...
- you already have F mount lenses
Sounds like a great reason to keep shooting SLR for another generation, until the Nikon/Canon systems mature more. You get two slots, better AF and longer battery life, and, if you're going to keep using your old lenses anyway, you're hardly saving on weight and size.
- you need a rugged camera because you sail, shoot often in the rain, face situations where heavy condensation can occur,...
I've shot the A7r2 in sandstorms, on the rim of active volcanoes, in polar conditions and in tropical rainstorms. It never died. The A7r3 and A9 are even better sealed.
That one test where someone put it under a shower isn't particularly indicative of anything. You're not meant to put these cameras under a shower without protection. It's like asking whether an A7r3, 5D4 or D850 makes a better hammer to pound in tent pegs.
Or, you know, keep shooting that D850. It's built like a tank and works well with your existing lenses.
- you value very high quality lenses that are compact on a compact body
Of which Nikon has announced exactly one.
Then there's the entire Batis lineup. And the new Sony 24/1.4.
- you prefer a better EVF than the very decent a7rIII one
- ...
So you'd give up a second card slot, a much larger lens lineup and a whole generation of AF performance... all for an EVF that one user said was slightly better and another said was about the same. Given that the two viewfinders have the same resolution, the fact that Sony makes so many EVFs and display panels and that Nikon's sensors are made by Sony, there's every chance that it's actually exactly the same EVF, with a different fitting/EVF optics over the top.
And we know Sony has a >5MP EVF ready, as well as the multilayer sensor technology (as demonstrated in the A9) to make them function even better than they do in the current generation. Remember, the Z7 is a year newer than the A7r3, and one-and-a-half years newer than the A9. It's
supposed to be better. But, evidently, it does not match up.
In other words you look objectively at the value being delivered.
That's exactly what I did. And the fact is, apart from the sole case of a non-action-shooting, amateur user who puts a huge premium on size and weight (almost to the exclusion of everything else) and who's happy walking around with a single, slow 24-70mm lens, it doesn't deliver anything that isn't delivered better by either a D850 or an A7r3.
