What if they are not so extensible, but there is just too much money put in their development? These are some examples of the most complicated software ever bui

What if they are not so extensible, but there is just too much money put in their development? These are some examples of the most complicated software ever built and never were just-for-fun projects. What you are asking is as operation system development with all well-know set of literature, starting probably from Tanenbaum.