Yes f/4 is plenty fast enough for me, if a RF 12-35mm f/4 L or RF 14-35mm f/4 L is released for a similar price as the EF 16-35mm f/4 it would be the first lens i'd purchase after switching to mirrorless.
Sorry to ruin your day, but this new mount means newer, better, but also
more expensive lenses, even at the same aperture (let alone shorter starter focal lengths or ones with a faster aperture, which are at a higher priority it seems).
And it also looks like that we won't be seeing native RF lenses from other manufacturers either.
I agree. I was with a friend who is a Nikon shooter the other day, and she is considering an EOS R specifically to use the RF filter adapter + wide angle EF lens, specifically to be able to use rear filters. As a Z7 owner, she was very jealous at how well EOS R worked with EF16-35L4.
It also got me thinking, why make native RF wide angle lens smaller? I mean, who complains that their 16-35 is too big a lens, right? So just take an EF lens, add a control ring, build the adapter into it with the ability to put in a rear filter, and call it a day.
For that matter... I wish they would do it with some of the other lenses too. It would just so great to invest once in a full set of rear filters instead of endlessly buying front ones. I wouldn't hesitate to spend $300 each on the best quality of a few of my favorite rear filters, if I knew I could use them on every lens I bought in the future.
Their design priority is to make
'better' lenses. That means taking full advantage of the shorter flange with the wide mount diameter (especially for wide-angle lenses), so there is actual glass where the filter could go, it would be way too big of a compromise to the optical formula (while also adding even more to the costs and potential problems).
So again, it is just a non-feasible option unlike with the super telephotos. And the option to use the older EF lenses will always remain if one really cares about these adapters with the rear filters. Re-modeling old EF glass makes the least amount of sense(they are not designed to be focus by wire like these newest ones), and while not ideal, step-down rings also do exist for the front ones as well.