You're correct, I really view the 28-70/2 as a collection of prime lenses. For my typical subjects, I don't find it restrictive. Many years ago, I was deciding between the EF 24/1.4L II and the EF 35/1.4L and I set the 24-105/4 to each focal length for a couple of days. I found that I preferred 35mm, and in post processing I was routinely cropping the shots from 24mm.
But for a travel lens, I would not want it to start at 28mm. Even though I'd have something wider with me (10-20/4 or 14-35/4), the extra width will often mean I don't have to swap lenses for a shot, something I prefer not to do especially walking around with family. Having said that, I did use the R8 + RF 28/2.8 as a walk around lens for a while, and it does well so perhaps I'm overemphasizing the importance of those extra few mm on the wide end.