There is speculation being circulated that Windows 10 will be a subscription based service similar to Office 365. According to various posts and published speculations, Windows will be free as an upgrade for the first year. Then, after Microsoft has enough feedback on which devices reliably can run the new OS, the company will shift to a subscription based model.
Whether or not these rumors amount to anything remains to be seen. However, it seems unusual that the company could somehow give away its main product for free. They have no significant hardware like Apple, which charges nothing for OS X anymore.
Perhaps we will know later for sure this year or further on in 2016. If the next generation OS is subscription fee based, then Welcome to Windows 365.