I recall them saying Windows 10 would be a rolling release and it would be the last one you ever had to buy. Could be wrong though. I don't pay much attention because I haven't used it since Windows 7. I don't have a link to back this up, just my hazy memory.
They are still conceptually different operating systems. There are users who are unable to upgrade because their hardware is deemed incompatible, and when Windows 10 support ends they will be left without updates. Microsoft no doubt have reasons for this, probably some good and some bad, but ultimately this is not the same as the "Windows as a service" that was sold to customers when they paid for the licence.
About the hardware compatibility, it's the same as any other software as a service, at some point you might need to upgrade your hardware in order to continue being able to use the software.
Here are three sources reporting that Microsoft employee Jerry Nixon announced it at Microsoft's Ignite conference in 2015. They announced the transition to "Windows as a service" model, and it is also reported that Satya Nadella described this as a "new era" for computing.
I don't have a link to a Microsoft source, or the original video/audio, but the sources are reputable enough that I'm sticking with my original statement.