If MS has plans to make and sell phones like the 730/735/830 (Newer phones) they cannot carry on using the Nokia name and it would make much more sense to call the Microsoft Lumia's, or just Lumia's, OK for a time you will have some phones with Nokia written on them but since the takeover they are MS phones and they just cannot carry on using the Nokia name.
The change is happening, so the only alternative to rebranding is to stop making every phone in their current line up and start again, or simply retool the machine that stamps the plastic, one of those makes common sense the other doesn't.