I think Nokia must not ditch WP7 now, they will lose even more credibility. They will never adopt Android (and obviously iOS) so atm WP7 is the only major OS they have left. It's getting better and the fantastic design of the Lumias is helping. Symbian is not an option anymore, despite it being the most complete OS feature wise and new updates which are taking it back to a good level. In people's minds Symbian is a dead walking, noone will like it even if it's actually good. I think they should stick to WP7 waiting for their next "billion dollar strategy", apparently centered around Qt 5, to become operative and just then maybe ditch WP7 if sales will still be low.