And the rewritten successor to Windows 3.11 shouldn't have been Windows 95 (and then Windows XP, Windows Vista/7, etc.). And the successor to Mac OS 9 shouldn't have been called Mac OS X. And so on.
Face it--this sort of thing happens all the time. The name is just a brand, not a reference to a specific codebase. KDE4 is not even the first all-up rewrite of KDE. KDE 2.0 was "almost completely re-engineered" from KDE 1. This happens to most projects eventually; some things can be fixed as you go, but others are more integral and require a change in the fundamental design. Either you fix the design (and port/rewrite) or the project stagnates and is eventually replaced.