Upload & Sell: On
I am not an optical expert, but over the last 30 years I have owned lenses by Nikon, Canon, Olympus, Pentax, Zeiss, Cosina, Schneider, Makinon, Vivitar, Tamron, Tokina, Samyang and yes, Sigma. Never owned glass by Minolta, Angenieux or Leica though, but I've owned alot of glass.
Now I have the greatest respect for the build quality of L glass, and many L lenses are wonderful optically. But it does not automatically follow that just because a lens has "L" on it it is the best of its type available optically speaking.
There are so many breathtaking assumptions here, don't know where to start:
"Let`s also consider what the equipment says about the photographer."
No, equipment says precisely nothing about the photographer. Its about the images they produce. And a good photographer can make great images using a $20 Holga with plastic lenses, in fact many do.
"L lens clearly demonstrates an unwavering dedication to pursuing the best image quality, truly the mark of a professional."
Pffft! Again, the mark of a professional is (hopefully) interesting photography (quite alot of amateur photographers produce more interesting work than many pros btw so the distinction is often arbitrary). There are many serious photographers using non L lenses, even within the Canon stable. For example unless shooting at or near wide open I don't believe 99.999% of people could tell the difference between the output of an 85 L f1.2 and the affordable non L Canon 85mm f1.8, even on the largest prints. This illustrates that often the L advantage isn't necessarily absolute optical quality, but interesting features not commonly available on consumer glass, like tilt shift, durability or weather sealing or very wide apertures etc etc.
L lenses are built extremely well, many of them have very good/excellent performance, but they don't hold a monopoly on serious photography. Judging photographers on whether they own L glass or not is nonsensical and rather naive.
It would be much more rational to look at all available lenses in your focal length from all relevant makers, and then to decide what balance of optical quality, build, features and system compatability and price is best for you. Now that may well be an L lens (I own 4, with great satisfaction) but it is possible the best choice for an individual photographer, pro or not, is a non L Canon, or independent lens maker.