Considering SIGMA has had a full frame 12-24mm since oh, 2005 or so, I fail to see how this lens could require such a high price. Obviously this will be much better, all optical designs have progressed and improved, and it's Canon, but $3K worth? It just does not fit Canons pricing at all.
It does behoove Canon to have an 11-24L, and the idea of it being f/4 is compelling. I have the 2nd generation Sigma 12-24mm, and it is a very nice lens. It's great at car events when you want to isolate a car, or if you want to get a nice interior shot. At 12mm, it's also a fun lens for fashion. Ask Lindsay Adler!
When I bought the Sigma, I was looking at wide angle lenses from Canon. They didn't have anything like the Sigma. The Sigma was exactly what I wanted, and had quite good reviews. I got it and have never regretted the purchase. Current price at both B&H and Adorama is $950. The Sigma is 12-24mm; the rumored Canon is 11-24mm. The Sigma is f/4.5-5.6; the rumored Canon is f/4. Are 1mm, one stop, and one red ring worth an extra $2K? Canon has a strong competitor in the Sigma lens. Is $3K a viable price point?