Unless they show that AF is working properly, they can be as optically phenomenal they want. I will not even think about getting another Sigma until my 50 Art is fixed and both it and the 35 Art performs consistently over time. To compete with a Great White, they have to do something they have never done before and I have my doubts that they will manage.
I have a sigma 70-200 2.8 HSM that I've used to shoot a ton of basketball and the AF works great. Their 120-300 2.8 is pretty much universally regarded as an outstanding lens, especially for the money. I've been looking forward to Sigma getting into the super tele prime segment for a while now because the Canon/Nikon offerings are obscenely overpriced and it would be nice to see some decent competition at more reasonable price points.
Sigma doesn't need to compete directly with Canon/Nikon super teles metric for metric, they just need to make them ~95% as good for ~half the price. Their target market isn't so much the rich dilettante who just automatically buys the most expensive thing or the working professional wildlife photographer (lol, all five of them), but more the hobbyist who wants to be able to take pictures of birds and airplanes and stuff without taking out a second mortgage. The overwhelming popularity of the Tammy 150-600 proves that there are a ton of people out there who want a lens that's "good enough", gets them the reach they need for their hobby with decent IQ, and doesn't leave them living in a refrigerator box under a bridge. If Sigma puts out a 500 or 600 f4 OS for ~6-7k I guarantee it would sell, even if its not quite as sharp wide open or the AF isn't quite as tight as the Canon version. Extra points if they rework their TC's to optically match. Even more points if they come integrated like the Canon 200-400 f4