They weren't. People weren't going to be using 50 year old machines running Windows 10. Every time you buy a new computer, that's a new licence fee in to MS's wallet. The vast majority of Windows licences sold are OEM ones that die with the hardware they are first installed on.
The main idea of only ever having one version of Windows going forwards was to stop the hold-outs and reduce support costs. The same people who didn't want to install previous major updates of Windows were also quite happy to install "service pack x" on their current version. The plan was that going forward, there would be regular service pack sized updates, and in fact, Windows 10 now is quite a different beast to Windows 10 on launch date (just look at how much more stuff has moved into the Settings app compared to day one).
However, once again, a decision made for quite sensible reasons has been overridden by Microsoft's marketing department. Nobody really cares what name the operating system they have on their computer is. The updated UI was already pencilled in for the next Windows 10 update, it just looks like someone in Marketing has gone "oh, you've moved the start menu away from it's infinite corner*, we should call it something different!".
* The reason the start button was in the lower left is to take advantage of the infinite target size according to
Fitt's Law. Original designs included some with centred options, but it was found in testing that the usability of those designs was lower than the aligned options. The same rule applies to the application control menu (top left) and window close button (top right). They've not ended up in those positions by accident.