In reality Microsoft can stop supporting your computer's _hardware_ much earlier than that, if your device maker stops supporting it (reminds me of Andriod phones):
> A device may not be able to receive updates if the device hardware is incompatible, lacking current drivers, or otherwise outside of the Original Equipment Manufacturer’s (“OEM”) support period.
https://support.microsoft.com/en-us/help/13853/windows-lifec...
There are different versions/branches of Windows 10 that come out every 6 months or so.
Some of the branches/versions are "Long term servicing branches" (LTSB) for enterprise environments, supported for 10 years. There are already two of those. https://en.wikipedia.org/wiki/Windows_10#Updates_and_support
For everyone else, the branches are supported for 18 months. https://technet.microsoft.com/en-us/windows/release-info.asp...
In reality Microsoft can stop supporting your computer's _hardware_ much earlier than that, if your device maker stops supporting it (reminds me of Andriod phones):
> A device may not be able to receive updates if the device hardware is incompatible, lacking current drivers, or otherwise outside of the Original Equipment Manufacturer’s (“OEM”) support period. https://support.microsoft.com/en-us/help/13853/windows-lifec...
And they've already stopped supporting some devices: https://www.computerworld.com/article/3216005/microsoft-wind...