Personally I'd be glad to give up 5.6 at 400 mm for the extra 100 mm if the price is in a similar range as the EF 100-400. As mentioned above, with an EVF and the R focusing system this is likely not going to be an issue for the typical user of this lens.
Was shooting at an airshow last year and 500 mm instead of 400 mm at the long end would have been handy. For some shots 100 mm were just wide enough so that a 200-600 would not have been an alternative. I had the 1.4 Extender with me, but apart from the fact that it is not very practical to remove the extender every time a wider focal length is necessary, I had AF accuracy issues (with a 5D4) so that I ended up shooting at 400 mm and crop in post.
A ~40 MP R5 with the 100-500 would have been perfect!
The lens is likely going to be a bit longer at 100 mm vs. the EF version (based on a very rough picture comparison with the RF 70-200 which is similar close to the mount), but probably only about 10-15 mm, which is not a lot and less than the EF-Adapter required for using the EF version on a RF camera. If Canon is using a similar construction as for the RF 70-200 (mostly very robust plastic, see Lensrentals' teardown), the 100-500 might even end up being lighter than the EF 100-400. We'll see...