You get what you pay for.Maybe sometimes, but I would argue that sometimes (certainly not always) buying Canon lenses gets you better quality but with often diminishing returns. Your return on investment is almost never 1 to 1. For instance, you might get +10% quality for +200% price; or rather, small improvements for big prices.
It's funny. I think that people who shoot with Canon glass usually (usually!) produce better photos than those who shoot with third-party glass, but I think there's a selection bias going on as well. People who shoot with high-end glass are usually more into their photography, and generally more skilled and dedicated. Truly dedicated photographers choose invest in great gear to get them that little bit of an edge, but that doesn't mean if they had shot with slightly inferior glass most of their shots would have been different.
I agree with what you are saying, however the bias may well be due to the difference between doing photography as a profession, or as an amateur. It can account for the bigger budget allocation, as well as actually growing more quickly in your skills as you spend your time earning a living with it.
well, its all speculation anyhow I think that certainly there are some very nice 3rd party lenses, and especially as an unpaid photographer, its worth shopping around. Lets not forget cheap canon lenses can be just as full of duds as low end 3rd party lenses, its just that we often compare cheap 3rd party lenses to L glass, which is somewhat unfair.