I think Sigma has started on a trail that will be a lot more threatening to Canon than a Nikon 800E or a Sony A7r. They are going straight to the heart of Canon supremacy and offer top notch lenses at half price.
I sort of agree and sort of disagree.
I agree that third party manufacturers are a bigger threat than competing camera makers. But, I think the price of Sigma's "art" series raises real questions about how effective they can be in undercutting Canon.
They may be offering top notch lenses, but not really at half price. (Although they are discounted significantly from Canon).
This really comes down to the perceived value of the Canon brand. I wish Sigma well because I think it benefits all of us as consumers. But I wonder if they are pricing their products too high. What is the price premium that people are willing to pay for the reliability, durability and predictability of the manufacturer's brand over a third party?
Clearly the "art" series is going after consumers with a high amount of discretionary dollars and I wonder how many of those consumers are price-sensitive. I think it is a gamble and I wish them well, but I don't think there is any guarantee that their products will find a market.
Tamron's strategy, I believe, is more sound – Offer good quality lenses at very competitive prices and exploit available niches. In addition to the 150-600 zoom (which could well be the best selling lens introduced this year), they produce a reasonably priced 70-300 that vastly outperforms any of Canon's consumer grade 300mm zooms. And, their superzooms fill a niche that Canon doesn't even attempt to compete in.
Actually, if I were Sigma I would be very worried about Tamron and Tokina, which generally produce better quality at less cost. Indeed, that may be why Sigma seems to be going "all in" on the Art series.