Windows will be the default until suddenly it isn't. Valve is doing amazing at destroying the core of Microsoft's support. This story would be different if this was a decade ago, but these days most average people do their computing on phones and tablets. The ones sticking to traditional PCs are mostly gamers and now more than ever Linux is a viable alternative to Windows. Vanishingly few games can't be played perfectly fine on Linux. Once enough gamers are using Linux it will become the default choice, and once it's the default choice for gamers it will become the default choice for most people, at least the ones not on phones and tablets.

It actually wouldn't surprise me to learn that most Windows installs nowadays aren't pre-installs but rather images deployed from a corporate IT department.

In some ways the biggest danger for Windows in the home market isn't Linux or Mac but the people who decide they'll just use their phone or tablet for everything. Then again, I'm not sure if Microsoft even cares about the home market.