So many people think Microsoft will lose all Windows 10 profits and revenue, but they only said they're offering a free upgrade. They never said they'd stop charging OEM's a fee for each Windows licence they buy. Thus they may have a few hundred million hit on profits and revenues, but other than that we should expect good earnings for Microsoft. Plus we'll see a larger market share for Windows 10. Also they didn't say the business version is going to be free. And with the free upgrade, we should expect people to buy more Microsoft products (like more office licences or maybe an xbox live card with the savings). So in the end, it'll even out. And I'll repeat this it'll help it increase its market share of not only Windows 10 but against linux, mac, ect.
