There are many conclusions which could possibly be drawn from the experience of the Multics project….Third, systems of the computer utility class must evolve indefinitely since the cost of starting over is usually prohibitive and the many-year lead time required may be equally unacceptable. The requirement of evolvability places stringent demands on design, maintainability, and implementation techniques.
[1] “Multics–The first seven years,” Multicians.org, 2022. https://www.multicians.org/f7y.html (accessed Sep. 03, 2022).