None of the outsiders truly know the reason for Microsoft's purchase of Nokia. It could be for their products and services, their portfolio of patents, or a defensive block to prevent another company from doing so.
Microsoft has a long track record of re-re-re-rebooting their mobile operating systems. I've been using and developing for their mobile OSes since 2000. I'm painfully aware of how suddenly they drop and restart their mobile OS. Seems to be on a 4 year cycle. I see nothing in Microsoft's corporate culture to indicate that things will be different this time around. Especially since there is the inertia of project development... many times in a F500 company, projects must be carried out to completion (for financial, contractual, and legal reasons) even if the decision has been made to kill the project. The Kin was such a project.
Just to refresh everyone's memory:
Windows Mobile 2003 : June 23, 2003 (1st Windows-based smartphone OS)
Windows Mobile 5 : May 9, 2005
Windows Mobile 6: February 12, 2007
Windows Mobile 6.1 : April 1, 2008
Windows Mobile 6.5: May 11, 2009
Windows Phone 7: November 8, 2010
Windows Phone 8: October 29, 2012
Windows Phone 8.1 : April 2, 2014
That's not including PocketPC 2000 released in 2000 that was the foundation for everything that came afterwards.