What will happen to Windows 10 free upgrade after 1 year?

Windows 10 will be a free upgrade for Windows 7, 8 and 8.1 during the first year of its availability. This statement from Microsoft had given rise to speculations that there will be annual subscription for continued usage of the OS. But it has been confirmed that there will not be any additional fees for windows 10 once you buy it.
You will keep getting updates and support till the lifetime of the OS. There is no subscription model as has been speculated.
If you are on Windows 7, you can upgrade to Windows 10 for free in the first year. If you do not , you will need to pay for upgrading. The offer expires in one year, not the upgrade.

In simple words, after one year of free use of Windows 10, you have to pay a one time fee to Microsoft for Windows 10. After that, Windows 10 will be yours forever. And you will get lifetime support/updates of Windows 10 without any charge.

You seem to have misunderstood the offer. Windows 10 will be available free for upgrade to Windows 7, 8 and 8.1 users during the first year of its launch. If you upgrade within this offer period, you will not need to pay anything even after a year. If you don't upgrade within the first year, you will have to pay for your copy of Windows 10.