And I strongly disagree with the title that MS is the one holding secure web back. I'd blame the users (both corporations and personal ones) who won't upgrade, either to modern browser or a modern OS.
edit: Additionally those nice graphs are misleading. I bet other (SNI supporting) browsers have significant market share on XP too. Compare the market share of IE <9 (<20%) to XP (33%). And some of those IE8 users probably are on Vista/7.
edit2: And if we look at IE8 market share trend, it has been steadily decreasing for the past year. I feel safe to say that in a year it has decreased to a low enough percentage so that it can be ignored.