Microsoft announced the upgrade strategy of Windows 10 in their January consumer press conference. If the promotion is claimed within the first year, the new operating system would be a free upgrade to users of Windows 7 and Windows 8.x “for the supported lifetime of the device”. The internet combined “the first year” with “for the supported lifetime of the device” to speculate that Microsoft would charge a license or subscription from then on.
The bandaids will keep coming until 2020 (features) or 2025 (security)
Now, less than two weeks before launch, Microsoft has released an official statement on the matter. Windows 10 will be supported until 2020 with extended support reaching out to 2025.
This does not mean that every device will be supported until then, though. In order to receive new updates, the user needs to install every update prior to that update, because they are cumulative. If a device is incompatible with an update, and the affected manufacturer is not willing to fix the problem, then Microsoft will not make a special support branch just for you.
But if an OEM has stopped supporting a device, yet Microsoft updates don't break anything, then you can still receive updates for Microsoft. Again, the clause is just for the case where Microsoft's updates are incompatible with your device — it is not Microsoft's responsibility to keep giving you updates for the manufacturer's problems until the OS gets end-of-lifed.
This does lead to some concerns, though. Imagine that Microsoft pushes a build which disables developer sideloading and Win32 support, locking Windows 10 down to a Windows RT state. If you are even capable of blocking that update, Microsoft will likely not allow you to receive security patches. On the other hand, I don't exactly blame Microsoft for branching the operating system for each individual milestone. Doing something like killing support for Win32 would also be pretty brazen, although it looked like Windows 8.x was on that trajectory when it relegated the Desktop as an “app”… and one which all Win32 software required to survive.
So freedom advocates will have some things to think about, but those concerned about “free as in beer” can sleep a little easier. They will continue updating you for as long as the hardware supports it, until at least 2020 for features (at least 2025 for security). Beyond that time, it depends on whether Microsoft will charge for upgrades to whatever the next version is. Will it be a free milestone (think service pack)? Or will it be classified as Windows 11 with an upgrade or subscription fee? Who knows.