> 3) “Code churn” is a very uncharitable description of day-to-day maintenance of an active codebase.

Also implicit in the discussion is the fact that Google and other big tech companies performance review based on "impact" rather than arbitrary metrics like "number of PRs/LOCs per month". This provides a check on spending too much engineer time on maintenance PRs, since they have no (or very little) impact on your performance rating.