I am wondering how this would compare with the RF 70-200 F2.8 at 135mm (f2 vs f2.8 aside). At least for my use case, I find that the IQ of the 70-200 f2.8 is not that different from the RF100mm f2.8. So apart from the latter's micro capability, the zoom can serve as well in most of my situations. I imagine the 135 f2 to be mainly used as a portrait lens, but not sure (yet) if using the 70-200 could be good enough.
That's an interesting point. There was a Sigma CEO interview from a long ways back where the fellow indicated their 135mm f/1.8 lens wasn't selling as well as the other Art series lenses, and he attributed it to the fact that the 70-200 lenses (there was a profusion of new ones that had come out around then) were so good, people thought it redundant.
I own that Sigma lens, and it's just amazing. For event shooting and shooting woodcock in the evening from close up, it's hard to beat. The Sony attempt at that configuration is reputed to be even slightly better, although I haven't shot it.
In terms of competition versus the 70-200s, I think it's true for non-pros. For some pros - the ones who concentrate on low-light genres - the brighter 135mm lens does matter.
After you get a few hundred great woodcock shots at 8 p.m. over the years, you start to care about the ones that are done at the lower ISOs. You get more selective, keeping the ones where the bird landed within a few yards of you. It's so dark that the difference between a simple keeper and one that you'll run by a photo editor is the one you can take at 1600 ISO at 1/15th of a second versus the one that needed ISO 4000. Normal people - even most pixel-peeping forum dwellers - aren't going to sweat the extra stop enough to shell out $1,400 (Sigma) or $2,xxx? (the new Canon).
I think back in the day, when the prime was expected to be significantly sharper and one stop brighter, you might think of the zoom as a wedding photographer or journalist's lens, where the 135 was a professional portraitist's tool. The quality increase in the zooms has made that quite a gray area now.
Two days ago, I took this picture of the first landing of the first woodcock of the year. It landed about 10 yards away, and this is at 200mm with the RF 70-200 f/2.8. It was taken at 7:30 p.m. at ISO 6400 with -2/3 of exposure compensation to allow the shutter speed to be as fast as 1/20th. I looked at it on the computer, and the very first thought in my head was "should have used the 135."