The relentless onslaught of changing requirements that any successful system attracts can gradually undermine its structure. If such sprawl continues unabated, the structure of the system can become so badly compromised that it must be abandoned. As with a decaying neighborhood, a downward spiral ensues. Since the system becomes harder and harder to understand, maintenance becomes more expensive, and more difficult. Good programmers refuse to work there.
Investors withdraw their capital.
And yet, as with neighborhoods, there are ways to avoid, and even reverse, this sort of decline. As with anything else in the universe, counteracting entropic forces requires an investment of energy. Software gentrification is no exception. The way to arrest entropy in software is to refactor it. A major flood, fire, or war may require that a city be evacuated and rebuilt from the ground up.