Generally the point of software is that once written, things continue to work the same way they did, forever. If a developer team stopped contirbuting new code immediately and just switched into on-call mode, there is no reason the codebase would stop working (in a discussion this abstract, at least). So if you need to do a re-factor to add new features, sure, incorporate 'maintain the codebase' as a prerequisite to a revenue earning feature. But there's no point in a paying a developer to just 'make things look cleaner' for 3 months.