Personally, the f/2.8 is my 2nd-most-used RF lens after the 100-500. Not sure an extra mm would do much for me. If I need to get super wide, I'll use a prime with a much faster aperture. The Sigma 14mm f/1.8 is still great as adapted.
Seems there will be something other than 1mm and 1 stop to distinguish it from the 15-35. Like a substantial reduction in cost, weight, or size. Otherwise it will be rather redundant.
Personally, the f/2.8 is my 2nd-most-used RF lens after the 100-500. Not sure an extra mm would do much for me. If I need to get super wide, I'll use a prime with a much faster aperture. The Sigma 14mm f/1.8 is still great as adapted.