[yowlingcat]

I have found it's more practical to invert this train of thought and work backwards. In most codebases I've worked in, there are fairly clear divisions of labor between vanilla line of business code and critical path code that merits extra infrastructural investment.

Working backwards from the critical path code to reinforce it with the infrastructure it needs to support robustness and ease of maintainability is valuable because it makes the path to ROI for the business very clear, and candidly, there is often lots of code that just wouldn't be high ROI to reinforce too much.