This most likely means that Microsoft is aware of the possibility that Nokia might fail in staging its comeback.
While I think that the Nokia plan makes sense, it does not mean that success is guaranteed. Nokia might simply run out of cash before the plan is executed and may have started way too late after years of internal neglect at the hands of executives too comfortable with the permanence of success.
I think the most likely scenario if Nokia fails is that Microsoft buys the remains of the company to build their own hardware shop for handsets and tablets. There are also other valuable assets within Nokia such as Nokia Maps and a vast patent portfolio in addition to great knowledge about hardware.
If this happens, it will be one of the larger mergers in Microsoft history but I think it will make sense. Today they don't have that many hardware partners lined up for Windows Phone products and hence not that many to disappoint. I also think that the post-PC devices such as smart phones and tablets are more suitable for hardware - software integration and may lure Microsoft to change its decades old strategy of OS licensing to OEMs.
Microsoft smart phones, tablets and X-boxes could be a good re-start for the company which has seen its reputation wane during the past decade.