They can’t charge you for a new version if they keep updating the current one.
They'll figure out how to charge you, have no fear there.
But at the same time, they finally recognized that they'd introduced so many problems over the years with each new major release, that they decided, "No new major releases, just updates" so that users would still see the same version name ("Windows 10") and think "Oh, it's still the same, just updated".
In fact, the semi-annual "updates" have been in most cases major releases. But the trick has worked -- people think "Windows 10" and don't pay much attention to the version.
The fact is, as long as Microsoft leaves the desktop UI looking the same, they could substitute a pile of high-speed rabbit doots under the hood and people would still think it's Windows 10.