The trouble is they have a empty space between Symbian and WF7. People bying Symbian today might get annoyed, when it get dumped in favor of WF7.

That situation can be resolved in one of the following ways:
1. continue support for Symbian for the time that is needed to perform transition of 70% of customers
2. provide WF7 (or other) firmware update for limited number of Symbian phones
3. dump Symbian phones no matter the consequences.

#3 is most likely, and it will make them drop from #1 (some may argue they are not #1 anymore).

The thing is that they need to keep existing customers somehow, so they can have eco$i$tem. Saying "we are nokia" is not enough.