I would like to suggest that there is a fourth dimension that might be called 'interest' as we are using a debt analogy - the tendency for the cost to increase over the time elapsed since the debt was incurred.
When an item of debt is first created, the people making it are often well aware of what they have done and are therefore in a relatively good position to fix it, but that knowledge quickly dissipates, to the point where it is often forgotten that there is a specific issue there. Furthermore, there is a tendency for it to be made less obvious as further changes are layered on top and around (this is distinct from contagion, as it can occur if the later changes are themselves debt-free, or at least independent of the decisions that created the debt and their consequences.)
One place I worked addresses this by having mandatory post-deploy monitoring / patch day. We’d all do a deploy and keep an ear to support / logs while going ahead and improving things we knew needed a little clean up. If we saw anything come in from the release, we fixed it immediately.
An entire day is excessive in a CD setup, but for a two week release cycle it worked well. Kept the rough edges out of customer view very well.
The top comment under the articles uses the hight of the interest rate to describe the level of contagion http://disq.us/p/1ros2o9
'tl;dr "contagion" is the most important attribute because its properties are similar to interest rates. Having a small loan (small impact/fix cost) but high interest rate (high contagion) can quickly dwarf large loan small interest rate.'
When an item of debt is first created, the people making it are often well aware of what they have done and are therefore in a relatively good position to fix it, but that knowledge quickly dissipates, to the point where it is often forgotten that there is a specific issue there. Furthermore, there is a tendency for it to be made less obvious as further changes are layered on top and around (this is distinct from contagion, as it can occur if the later changes are themselves debt-free, or at least independent of the decisions that created the debt and their consequences.)