Why Nikon and Canon should build mirrorless as a second system - but probably won't

All rumors suggest that Canon and/or Nikon is going to get into the high-end mirrorless interchangeable lens camera market, in some way. I don't have any insider knowledge of this, or I wouldn't be able to write this, but it looks pretty likely at this point. It also seems pretty likely to me that both brands will do everything to maintain compatibility with their existing lens mounts. Avoiding the need to design a whole new lens range, maximizing the value of the investment in the existing line...