Factually, Windows 10 is probably more like Windows 8.1 than Unity 5 is like Unity 4 (if you measure % of code change/added – Windows is a much bigger codebase, of course!)
The core of this sub-discussion is this: The claim that “UE4 is a new engine” just doesn’t hold water.
UE4, itself, has been used in shipping games for several years, and it traces its direct lineage all the way back to original Unreal, just like Unity 5 traces its lineage back to Unity 1.
Whatever the differences are, “age of engine” isn’t a major one.