Is there really room in the market for 15" and 16" models? What will be the differentiation other than the screen size?
If the 16" is just the 15" with a slightly bigger screen as a result of shrinking the bezels then probably not, in the long run - could be a gradual transition to a 13" Air, 14" MacBook Pro and 16" MacBook pro line-up. If it has other additional features, maybe.
Secondly, take a closer look at the menu bar, there is already enough space there to account for a rounded corner on both sides.
There's certainly space in the UI for a slight rounding of the corners - but I think the sort of corners shown in the artists' impression would still clip it - especially if people wanted to use it in 'looks like 1920x1200' scaled mode. Ditto if they completely lost the bezels from the 15" design but kept the same outer radius of curvature.
Of course, Mac OS Classic used to have (software-)rounded corners and the world didn't end.
Why release a brand new version of the butteffly keyboard so close to the release of the "next gen" keyboard?
...possibly because the existing case was designed around the ultra-thin butterfly keyboard, so there isn't room to replace it with a thicker, scissor mechanism without completely designing the case (and re-tooling the factories). The new keyboard will have to wait for the next major re-design, which would be just about due next year anyway.