consider the market for this new camera. It's not those of us who have full dSLR and a kit of lenses. It's for the newbie moving up from a P&S who doesn't want the complexity or bulk of our favorites.
When the EF-S lenses were announced, along with the Rebel, you heard the same gripe. Why a new mount? Anyone still asking that question?
Canon has been in this biz a long time, and successfully stole the lead from Nikon some years back. I suspect they know a thing or two about the marketplace we don't.
While we're on Canon, it's interesting to note that they pioneered stationary mirror SLR around 40 years ago with the Pellix. Note that they haven't gone back to stationary mirrors even though others like Sony have. Curious. Wonder what Canon knows that the others don't?