Comment by jeffparsons

One way to communicate this is in terms of _schedule risk_, rather than straight-up lost productivity. E.g. for a particularly troublesome design flaw, you might estimate that for any given 4-week project there's a 20% risk of the project blowing out by an extra week due to that flaw.

Depending on the stakeholder(s) you're dealing with, approaching it like this (as risk assessment/management) might help to convey the short-term impact of a long-term problem.