I don't think that Nokia lacks strategic planning, but maybe you lack understanding of their strategy?

Honestly, none of the Maemo devices have been mainstream devices, so ensuring backwards compatibility would not be economically feasible. And actually backwards compatibility between OS 2007 and OS 2008 was better than you pointed it out. OS 2006 and OS 2007 was more of an issue.
Of course, from a technical point of view, not all decisions Nokia makes appear reasonable and some are purely political, but I'm certain they all fit into a big plan.