After the first year you'll receive a notification which reads, "Your device has reached the end of its support period. If you would like to renew your support license go to www.microsoft.com/...."
Microsoft has never clearly stated in any of their press releases that they will not be doing this. Every single press release has been nebulous double-speak designed to avoid declaring that this is exactly what they intend to do, and exactly what Microsoft executives have told shareholders when discussing wanting to turn Windows 10 from a product into an on-going service.
Your device will be supported for the duration of the support phase, and the duration of the support phase is determined by the duration of the support license that you purchase, not the planned lifetime of product updates.