If it's $1300 I likely will not buy it. In the rare circumstance in which my 35mm Art is just too wide, I'll settle for my Canon 50mm f/1.4. This may be mostly psychological but at $1100 I might just go for the 50mm Art.
Coming in at 1/3 the price of the Otus but with similar build quality, nearly-as-good optics and AF? If all that can be said, I think Sigma will get quite a few takers even at this price.
The only difference is that the Otus is cool and can be shown off, the Sigma can't. This is why I think the price will be < 1000 USD.
Shown off to who, if you don't mind my asking? It's such an exotic piece of tech that only other photogs would know what it is.
With that same money, you can you get a second FF body along with this new Sigma 50... Just saying.
I'm going to go out on a limb and say that Zeiss is counting on a few of the Ferrari-driving enthusiasts to pick up an Otus and say "hey, look at my pricey new toy", but more likely they are aiming to impress the best pros with the best tool in that focal length.