The mainline is an active branch, with regular drops of new and modified code. Keeping it healthy is important so that when people start new work, they are starting off a stable base. If it's healthy enough, you can also release code directly from mainline into production.

Refactoring at scale is challenging and a significant investment, so communicating the progress is important both from a business perspective but also as a motivational effect to developers. In this case we used a combination of complexity trend visualizations and Code Health Metric as shown in the preceding example.