>>Apple's OS is already cheaper than Win if you have to buy it.
Yes but Apple make both the hardware and software so get money from the very expensive hardware when you use their software products.
MS get no money from the hardware if you decide to put Windows on a PC.
Both Windows and Office have been "cash cows" for years for Microsoft so they cant afford to keep giving Windows away for ever (though the free upgrades to W10 don't apply to Enterprise customers so they will still get money that way).
However I cant see Windows being "free" for Home users for ever, unless MS expect to make more money from digital downloads in the future (games, films, TV shows, music etc) to subsidise the loss of revenue from Windows itself.
I realise the OS market is changing, with people getting their OS "for free" on their smartphone or tablet (including OS upgrades), but I still cant see Windows being free for home users.
Unless in the future MS ships a "core" Windows for free and you have to pay for extra features, that is one way to make money.