[Joeri]

Actually, a large codebase can be reasonably expected to live 20 years. Over that time with a lowish 10% turnover you'll replace the entire programming staff twice, so the understanding people have of the code is not just hearsay, but hearsay of hearsay. The sheer size of the codebase (facebook has tens of millions of lines of code) means that practically speaking you can't program your way out of a corner once you get there. That is (partly) what killed facebook's competitors. The solution to all of this is to be ruthless about simplifying code. The goal of new code should first and foremost be to be maintainable. Clever code is the enemy of scaling a programming team. Not that you don't need clever code, but you isolate it and protect it and make sure the average team member doesn't have to look at it.