On one hand, this would take me down to only 2 EF lenses in my bag - the Canon 85mm f/1.4 IS and the Sigma 28mm f/1.4. On the other hand, as others have mentioned, my existing Sigma 135 f/1.8 is superb, and if the RF is "priced premium", it begs the question as to whether it makes more sense to get the RF 70 - 200 f/2.8 instead... A lens I was resisting due to the fact that I have that 85, a 24 - 105 f/4, a 100 - 500, and I don't shoot a lot of sports limited to 200 (else I would have chosen it over the 100 - 500 to begin with). Being unable to use TC's with it makes it very narrow in purpose for me.
Filter thread size is also a concern; everything in my bag is 77mm or less, except the 15-35 and that Sigma 135mm which are both 82mm, so I have standardized on 82mm filters. I would love to be at 77mm instead, but I got the 15-35 before the 14-35 was available, and in retrospect I love being able to get down to 15, have f/2.8, have IS, and have minimal optical distortion compared to other Canon RF ultrawides; it's literally the lens I use most. If this lens would be bigger than 77mm but not 82mm, I'm out.