"Software projects need to evolve over time, but they also need to avoid collapsing under their own weight. This balancing act is something that most programmers understand, but it is often hard to communicate its importance to nontechnical stakeholders. Because of this disconnect, many projects operate under the false assumption that they must stagnate in order to stabilize.
"This fundamental misconception about how to maintain a stable codebase has some disastrous effects: it causes risk-averse organizations to produce stale software that quickly becomes irrelevant, while risk-seeking organizations ship buggy code in order to rush features out the door faster than their competitors. In either case, the people who depend on the software produced by these teams give up something they shouldn't have to."
Guidelines for growing a stable codebase covers six useful techniques for keeping your codebase stable while it increases in size. Well worth a read.