The key to this dilemma comes down to one word:
"Microsoft will face an unenviable choice: Stick to plan and put millions of customers at risk from malware infection,"
I don't think that Microsoft actually considers these people "customers." I think MS very distinctly considers them non-customers of their flagship product, since they have not purchased any of the four latest versions (Vista, 7, 8, 8.1). All of Microsoft's customers should have followed its exhortations over the last five years to spend a few bucks and upgrade dump their now-13-year-old OS.
It's indisputable that across the computing industry, the perceived mandate of legacy support for next-gen OSes is increasingly feeble. In non-desktop markets - e.g., consoles and phones - the presumption was never there to begin with (starting with the Super Nintendo!) Web programming exhibits similar tendencies - how many Java applications from back in the day won't run on modern browsers? And won't that include the entire Silverlight platform in a few years? The tendency is that the river of upgrades will carry all projects of significance along in its current, and the projects that gather on the banks (i.e., don't receive newest-OS upgrades) are... detritus. For right or wrong, that's the view.