Imho, you don't jeopardise a successful line, which the 6D seems to have been, and Canon hope the 6DII continues that line to be, by making such a huge change from DSLR to mirrorless. If Canon seriously go mirrorless then they should make equivalent models to their DSLRs to give people the choice. It may mean some duplication and overlap of models but then if Canon don't have the resources to do it, then nobody does.
Saying that, Canon will be hoping that the 6DII is such a success as to stave off the need transition to mirrorless a bit longer, and for them to get the tech in place to make the transition easier.
From the reviews so far, I don't think the 6DII will be as successful as the 6D, even taking into account the declining camera market.
If anyone wants mirrorless atm, especially FF, then Canon (and Nikon) is not the place to look, other companies are giving better options, and some will even take Canon (and other manufacturers lenses) though they may not work optimally. Depends on how lenses invested the individual is, how much they want to continue to use said lenses, and what compromises they are willing to put up with on whatever camera they choose to put the lenses on.
In an ideal world Canon users would have a Canon FF mirrorless option that will take their Canon lenses. If they want mirrorless of course, and is has to be said again, most people still do not want mirrorless ahead of DSLRs, which are still outselling mirrorless cameras, which is why the 6DII is not mirrorless.