I think you're imagining much larger rewrites. A system such as Gmail is composed of many smaller parts. If one of those parts was written years prior for a wo

I think you're imagining much larger rewrites. A system such as Gmail is composed of many smaller parts. If one of those parts was written years prior for a world that has since changed, it may be accruing technical debt as it's continually extended to fit new requirements. An occasional rewrite helps address this type of decay. Without the rewrite you may find yourself 10 years later with a system that's both critical and kludgy, and at that point the rewrite will be a much larger project.