As a fashion shooter for the past 37-years, I can safely say that C1 is not the only program used by fashion shooters. I started using C1 when it was first introduced to process the original Canon 1D files. It was the best conversion program out there at the time. Times change. For conversions, I find it more convoluted to process RAW files than in earlier years. Tethered, it's "OK", but I find it slows me down. (Lightroom is even worse when tethering, except still-life. For both, this is because of the "overhead" required to convert RAW on the fly while you shoot. I have issues with both crashing when shooting tethered quickly.)
In reality, for my workflow, EOS Utility is far more stable, and I shoot RAW+JPEG. You may wonder why? The camera is doing a much faster job on the fly than the computer. I also don't believe in doing "refined" TIFF images for every frame I shoot when I may shoot 300-400 frames on a shot and the client wants one final image. Most of my clients prefer I do a final conversion of their select, and those who don't, prefer to work with the in-camera JPEG while having the RAW as backup. I also have never found that C1 does that great a job with skin tones. I prefer the rendition from Lightroom. In a way, you can liken it to issues I had during the film days. Kodak EPN (what a crappy, flat film) was "more accurate", but in the end Fuji Provia had more pleasing color. The other advantage for Lightroom is the ability to create a custom camera profile with the X-rite Passport, whereas I have to rely upon C1 to give me what they have created.
The one area you can say C1 has become the "standard" is from the standpoint of tethering MF cameras. There just are no real alternatives if you tether, and as such, it is a captive market. Most of my work is on location, so I only tether for studio shoots. All these comments aside, every time C1 comes out with a new version, I test it to see if tethered shooting is any better than previously demonstrated.