It just seems like there is a lack of healthy competition in the world of lenses. Since Canon and Nikon (and Sony) are mutually incompatible on lenses (without employing some Frankenstinean adapter and losing AF), they don't compete directly on lenses. They compete only as entire systems. And once you have bought into one "system", it is terribly difficult to buy into another whole system.
Back when Ma Bell had a monopoly on long distance, it cost a fortune to make a long distance call. Then they started competing a little when alternate providers like MCI and Sprint appeared, and then long distance got cheaper AND BETTER. And then cell phones came along and started offering free calls to anywhere and basically made the word "long distance" obsolete, while again improving quality and drastically reducing price.
Personally I'd LOVE to see some of that competition enter the lens world. Sigma seems like it may be "first among equals" of the second tier/3rd party lens manufacturers, and they actually make cameras too, so maybe they have motivation to step up a level on lens IQ and QC. Maybe they could stand the best chance of providing some competition.
Even if I never buy a 3rd party lens, the mere existance of good quality competitors might keep costs somewhat in line, and to promote continuing innovation/improvement among the big boys too.
Or not. Maybe it is naive optimism here.