Windows 10 may well be the 'last version' of Windows. Microsoft has announced that it will be moving away from the current model of a few large releases, to more frequent rolling OS updates.
As TechCrunch notes, "Microsoft has, especially under new CEO Satya Nadella, made the move to become much more of a services company. Putting Windows into that bundle of services would be a strong move toward upping the overall value proposition of the whole".
While we lose the 'excitement' of the announcement of a new version of Windows every couple of years, we'll benefit from receiving much more frequent updates so that new features will reach your desktop much sooner than they would otherwise. After all, as Geek put it, "Waiting years for a company to tweak something feels so 20th century".
However, the question as to how any future Windows updates will be charged remains unanswered.