I don't have my copy handy, but he was explaining that the great cathedrals of Europe (some of which took hundreds of years to complete) were only possible because generations of architects were willing to subjugate their own sense of aesthetics in order to maintain the original architect's vision.
If each successive architect decided to start from scratch, the building would never have been finished. And if each decided to build their part consistent with the styles of the time, the building as a whole would be a mess.
His argument (IIRC) was that programmers need to adopt the same mentality: overall consistency in the design takes priority over "doing it your own way", even if your way would be better (if the whole system were redone from scratch).