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And your assumption, that nearly all lenses look like this, is completely wrong. The 50MP is the only lens, that shows this behaviour. No other lens, I own, shows this. And you can't tell me, that it is the most difficult task in the world to develop a 50mm lens with sharp corners.
I didn't say "ALL" lenses look like this, I said 50mm lenses (but generally wider lenses also). You should read what I write, as well. I would say show me a 50mm that isn't soft in the absolute corner until f5.6 or f8. They all use pretty much the same type of design (Planar). I have 8 or 9 50mm lenses and they all show this. I would also note that the degree of performance in the corner depends in part on the focus distance. This is probably why users have commented that this has not been a problem in their images produced by the 50MP.
If you are keeping the lens, then I'm really not sure what the point of all the obsessiveness is with the performance in the furthest reach of the corner. We know this is a characteristic of the lens, easily understood from the MTF. Continuing to harp on it doesn't affect it, or make Zeiss run to their lab and change their excellent design. Is there a point to all this effort, beyond what we already understand?
Is it "difficult" to produce a 50mm lens that is sharp and has a perfectly flat field across the entire frame at all apertures? Probably not. It's probably prohibitively expensive, and would be at least twice the size and weight. Even $4000 Leica 50mm lenses don't do it. Correcting that final 1% of performance in the absolute corner would increase the cost of the lens several fold, and produce a lens that you would not want to carry. Most users wouldn't buy it, and prefer to accept optimal compromises and tradeoffs. With the 50MP, Zeiss has kept performance strong across 98-99% of the frame and then lets it drop off steeply into the absolute corner. Other lenses may choose a slightly different tradeoff, where performance drops off more gradually towards the corner. With this you will see compromised performance by the edges of the frame but it may preserve a bit more in the corners (depending on how well it is implemented). The 35-70's tradeoff is like this, it is pretty much downhill across Zone B and into Zone C of the frame (but it's also not a Planar-style design). Quite frankly, I think Zeiss' tradeoff with the 50MP is optimal. They have kept the price reasonable and the the size of the lens usable and sacrifice only the least important (by a wide margin) tiny fraction of the frame -- a fraction of the frame that only rarely you see in a print.
It is, of course, much easier to do with a 100mm focal length, and that is why you see the difference between the 50MP and the 100MP. Of course the 100mm won't do when you need or want a 50mm, and perhaps is one reason why Zeiss makes both.