Experience
Especially with a code base that is mature, assumptions correctly made years ago can be terribly difficult to deal with later when the current and new assumptions reign. Writing code that lasts even 5 years and "is robust" is what everyone wants to do for sure, but the recipe for doing just that is not easy to learn and even harder to actually do. For example, you might know what changes need to be made to conform to even the most obvious object oriented design principles, but the business requirements and time to market needs dictate leaving once again old crusty code alone and racking up yet another round of technical debt. Over time, this technical debt will demand payment and the effects on your ability to hire, employee morale, design changes possible, speed of delivery, testing burden, marketing message etc. become very real and very painful. I wonder, can these concepts can be fully learned without actually experiencing pain? Could you even attempt to learn these concepts experientially in college? My experience so far makes me think that one may know something intellectually, without really knowing it. It seems like for so many, one may talk about design patterns or abstraction or low coupling, but until you actually try to build something the other way, the painful way, you just don't appreciate what you are avoiding. What's worse, junior developers who, for no fault of their own lack experience with "the hard way" have a difficult time understanding why one must "go the long way around" to do what seems like such a direct solution. Passing on the stories of the past and their consequences and lessons seems to be an unending challenge.
From my economics book comes this fictional story which I think illustrates the point:
"One Pepsi plant is managed by an economics major with an MBA and has a labor force with an average of 10 years of experience. This plant produces a larger output than does an otherwise identical plant that is managed by someone with no business training or experience and that has a young labor force that is new to bottling."
I definitely fall into the "young labor force that is new to bottling" camp. In the technology industry there is a tendency to glorify the young and bright rather than those who have the experience and have paid the price to learn the lessons that matter in the long run. Whenever I talk to some developer and we talk about where they learned some valuable lesson about software development, rarely do they refer to a book. Almost invariably, they say, "I worked with Joe on this project and he showed me x, y and z. I learned so much working with him." Smart developers, experienced developers who know how to teach and share important lessons to junior programmers seem like a key to the experience problem. Sadly, junior developers who have the presence of mind to ask, listen and apply what they hear are hard to find, and senior developers who are both willing and able to teach effectively are even more rare.
Perhaps the answer to all this is simple. Perhaps it's just he or she who writes the most code, wins. What I mean by this is simply when one writes a lot of code that increases the probability that he or she will make more of the key mistakes needed to learn how to write software with longevity. Just getting more exposure to "how bad things can get" helps bring a sober reality to each line of code written thereafter.
What's remarkable about all this, is that failures in teaching and learning are what keep us "discovering" new ideas that are 40 years old. Truly, "there is no new thing under the sun."