How can you tell for sure?
You may not have read my (long) reply elsewhere in this thread, but one of the things I said, and always sat, is that "nothing can be more potentially misleading than an improperly done test".
So you have a guy who walks into an even where Zeiss is showing the Otus line. He takes one, mounts it, takes a handheld snapshot of someone, then changes to the Nikkor, and repeats it. Note that a) the model has moved, b) the lighting is thus different, c) the focus on the Otus shot is missed.
And from THAT you want to make an absolute statement that the Nikon can hang with the Otus? Really? He did shoddy testing, so therefore, you can't take his results seriously. At all, His test, and therefore his results, become invalid. Case closed. Discussion over. If one decides to test, they need to do it properly, not quick cowboy style so they can rush off to a blog to write something to make them feel important.
Now, on the other hand, there really aren't many "awful" lenses today. And there always will be a lot of expense related to the "very best" versus the "pretty good". I've shot over 55+ Nikons including most every wide, normal and semi-tele in the AIS and AF-D line, and I've shot the two longer Otii, but not the 28. I own a mix of Nikon, Sigma ART, and Zeiss glass, all modern designs, and all of my choices were borne from exhaustive, proper testing (I test for a living) with the idea to match the lenses properties to the tasks I need them for. So I know what a lot of glass shoots like. And of course it all depends on where the bar has been set for image quality. 1024 pixel wide web images on some site that mangles the hell out of your jpegs? You're right - there would be absolutely zero point to an Otus. Probably zero point to even the old 28 AFD on a DSLR honestly. But on the other end of the scale, a DSLR shooter who prints large landscape prints, well, they sure as HELL are going to see the advantages of the Otus, because it's going to have great corners and edges while the Nikon doesn't, and have less veiling flare, better astigmatism control, and likely far better control of the color aberrations. And when you are printing large and excellence is your goal, then it becomes quickly clear that the Otus will get you there and quite possibly no other wide angle lens will do as well.
Obviously there are many points between those two poles, but just because you or I may not have the bar set as high as the guy who does buy the Otus 28 doesn't mean it's not worth it. As with many things, it gets expensive quick to move the bar past a certain point of pretty good for those incremental gains, and every photographer will have a different take on whether his current glass is good enough, or not. A lot of times it depends on the desired end result...