There's an inherent complexity in the problem being solved.
And there is an "accidental complexity" in the implementation of the solution.
Throwing away everything, people typically believe that they can avoid handling a lot of the "inherent complexity." But typically there is a good reason why the inherent complexity was addressed in the previous version of the program, and there's a big chance that the new "from the scratch" designers will have to relearn and rediscover all that, instead of transforming the already existing knowledge that is encoded in the previous version.
For anybody interested in the topic, I recommend the number of case studies presented in:
"In Search of Stupidity: Over Twenty Years of High Tech Marketing Disasters"
See about new rewrite of Wordstar simply not having the printer drivers that the previous had, and also other features people already expected, leading to Wordstar's demise.
Or what Zawinski's names "Cascade of Attention-Deficit Teenagers" (search the internet for that, the link here wouldn't work!)
"I'm so totally impressed at this Way New Development Paradigm. Let's call it the "Cascade of Attention-Deficit Teenagers" model, or "CADT" for short."
"It hardly seems worth even having a bug system if the frequency of from-scratch rewrites always outstrips the pace of bug fixing. Why not be honest and resign yourself to the fact that version 0.8 is followed by version 0.8, which is then followed by version 0.8?"
Or an interview with Jamie Zawinski from Siebel's "Coders at Work."
... "even phrasing it that way makes it sounds like there’s someone
who’s actually in charge making that decision, which isn’t true at all. All
of this stuff just sort of happens. And one of the things that happens is
everything get rewritten all the time and nothing’s ever finished. If
you’re one of those developers, that’s fine because there’s always
something to play around with if your hobby is messing around with
your computer rather than it being a means to an end — being a tool
you use to get whatever you’re actually interested in done."
If one is able to cover all the complexity, and it is not destructive to the goal, the rewrite is OK. Otherwise, one should be critical to the ideas of rewrites as they could be potentially secretly motivated by simple (jwz again): "rewriting everything from scratch is fun (because "this time it will be done right", ha ha)"