It's pure speculation that the motivation for Secure Boot was to protect Windows market share.
Given that the biggest threat to Windows is, well, (old versions of) Windows - i.e. inertia - and given that old versions of Windows don't really run too well, if at all, on modern PCs (finally, at last, thank fuck for that, driver support for XP is falling off a cliff) - protect it against what, precisely?
Apple since 2000 is a once-in-a-lifetime company. You cannot help but be impressed - but I'm not sure that in the wider context it's healthy for the industry (or the economy) for a company to be sitting on that much cash.