Another motivation for building a new system might be that you feel that you've got the experience you need to do the job properly. One way to have gotten this experience is to have participated at some level in the unsuccessful development of a previous version of the system. Of course, the new system is not designed in a vacuum. Brook's famous tar pit is excavated, and the fossils are examined, to see what they can tell the living. It is essential that a thorough post-mortem review be done of the old system, to see what it did well, and why it failed.
Bad code can bog down a good design.
A good design can isolate and contain bad code. When a system becomes a BIG BALL OF MUD , its relative incomprehensibility may hasten its demise, by making it difficult for it to adapt. It can persist, since it resists change, but cannot evolve, for the same reason. Instead, its inscrutability, even when it is to its s hort-term benefit, sows the seeds of its ultimate demise. Certainly, the departure of these ramshackle relics can be a cause for celebration as well as sadness.