Has been said about Windows 10, "You own it, you have it, you're done & going forward it receives its Updates".
And, "No, it's not free just & only for the 1st year & then you start paying". You buy it & you do not buy another OS version in 3 yrs... keep on w/ Win10 & its Updates. We don't pay for Updates. We used to pay for (new) OSs repeatedly.
The word is 'service' not, 'subscription'! A 'service' comes w/ the Win10 Platform and is included w/ the purchase price. Anyway, here, from, someone else is what "Windows as a Service" means.
According to this post on Twitter by Gabe Aul, Microsoft's General Manager OSG Data and Fundamentals team, there is no ongoing fee for Windows 10. Windows as a Service just means it will be kept continuously up to date.