I think the "undiscovered scope" part is where most people trip. You can think of software and how it fits with its users purposes as having multiple layers of

I think the "undiscovered scope" part is where most people trip. You can think of software and how it fits with its users purposes as having multiple layers of features, interactions and details. The top few features usually each have a few sub features that you have to tailor correctly to your users work flows, each of these sub feature often have sub details, interactions with other features, corner cases, data formats or cross compatibility considerations. It's a fundamentally mathematical issue. Adding a layer of detail is an exponential proposition and the exponential function is explosive in nature. If your software only has 3 major features and each have 3 sub features, that is 9 things to consider. If each sub feature has 3 corner cases you need to get right, that is 27 things. And if each of these 27 things have 3 details that gets you to 81 considerations. The next level is 243. Each of the 243 points tend to take a similar ammount of time to plan out and build weather it's at the top or the bottom of the pyramid. As a piece of software evolves over the years, the intricate details at the bottom get sculpted out. The software's fit to its purposes can become very fine. The thing is, people tend to think of software in terms of its top 2-3 layers of details, its 27 most important features. The finer complexity is often not as visible and it's just difficult for humans to keep so many items in mind. This is true of any complex technologies. People tend to think of cars as machines that have properties concerning speed and direction and suspensions and breaking but rarely think of the complex chemistry of the fuel aeration and combustion process, the complex internal forces and velocities of parts in the transmission, the carefully tweaked metallurgic alloy work that has gone in each of the thousands of metal parts, the carefully chosen properties of the plastics and the rubbers, the thermal properties of everything, the hundreds of electronic systems, the analog circuits, the thousands of specification lines of many dozens of communication protocols for controlling all these parts, the entertainment system, acoustic properties etc, etc, etc. All these things took decades to evolve and refine. It's easy when planning a rewrite, to plan out the few dozen items visible at the top of the iceberg and underestimate the amount of important details hiding bellow. Sometimes, some of the details might not be important, but often times they are the essence of your business case and at the core of the favourable economics of your product. It's the years of knowledge accumulated in the subtle details that provide you with a moat and makes it difficult for your product to be copied. If you rebuild with mostly the top few dozens of features in mind, and only vague ideas about everything else, you are likely to be creating a commodity solution, one that your competitors will have a much easier time to copy than your older finely tailored solution.