But maybe it's possible to do seemingly good modular design which favors extensibility, but fails at contraction/extraction. I had some experiences in my formative (college) years that lead me to want to write things the way I did for those successful programs. This is certainly something that requires exercising. If it weren't being exercised, maybe we wouldn't have been so successful. As in, if I tried to make a good modular design and only 5-10 years later we contracted the program or extracted new programs from it, maybe it would have grown enough cruft to make that a challenge, but extension would have still seemed easy.
As I think about it more, I'm leaning towards the issue being exercising the option of contracting the program. If you only ever extend, it becomes increasingly easy to blur the lines between the layers of the system.