> In other words, in the absence of RoI measures, the percent of engineering time spent on value-add activities is a pretty good proxy for productivity.

It's a lagging indicator of productivity. If the team is consumed with fixing old bugs, it means that teams weren't actually that productive years back and were instead just exchanging "results" for technical debt.

Instead of creating a perverse incentive to create messes, what if they did something like create a ratio: "60% of work is new features, and 40% is maintenance"? And if maintenance becomes all consuming, could they look back and identify the problems with previous launches?