Most rewrites are unjustified, either because the old code is good enough, or because the guys in charge of the rewrite cannot deliver better (whether or not the old code is good enough).
But some are.
Arthur Whitney creates each version of the K language from scratch (he is now at version 7). They are not backward compatible, but every single one so far has been better on the metrics that he cares about (succinctness, speed, and revenue).
Everyone says that the Mozilla->Firefox rewrite was unjustified, but ... the Mozilla codebase looked like a dead end to too many people working on it (from first and second hand accounts I heard), it's not clear that it was viable to continue with the old codebase -- and, even if it was, it is unclear that Mozilla would have been better off doing that: Microsoft was the unstoppable juggernaut playing dirty. IE won not based on technical merit (although it actually WAS the better one come version 4) - but arguably because it was integrated into the OS. In fact, it is quite likely that we would not have firefox today at all if they did not start the rewrite back then.
C# is essentially a rewrite of Java; done for legal reasons rather than technical reasons, but IMO the result is a much better technical solution than trying to retrofit and comply with the (legal) legacy of Java.
But ... yes, most rewrites are unjustified and often end in failure.
But some are.
Arthur Whitney creates each version of the K language from scratch (he is now at version 7). They are not backward compatible, but every single one so far has been better on the metrics that he cares about (succinctness, speed, and revenue).
Everyone says that the Mozilla->Firefox rewrite was unjustified, but ... the Mozilla codebase looked like a dead end to too many people working on it (from first and second hand accounts I heard), it's not clear that it was viable to continue with the old codebase -- and, even if it was, it is unclear that Mozilla would have been better off doing that: Microsoft was the unstoppable juggernaut playing dirty. IE won not based on technical merit (although it actually WAS the better one come version 4) - but arguably because it was integrated into the OS. In fact, it is quite likely that we would not have firefox today at all if they did not start the rewrite back then.
C# is essentially a rewrite of Java; done for legal reasons rather than technical reasons, but IMO the result is a much better technical solution than trying to retrofit and comply with the (legal) legacy of Java.
But ... yes, most rewrites are unjustified and often end in failure.