I think if Apple ever decide to produce say a 15" version of the MBA, then I can imagine it cannibalising sales of the MBP.

Apple aren't bothered in removing things if they have a vision for their future. The past has proved that! Remove the optical drive from the MBP's, and you have a thicker Air. And we all know Apple only do thin lol

Whether this is the right thing for each of us individually is another matter, what Apple wants to do is what will happen regardless.