Try framing it as interest payments:

When you get X customers, you'll have to commit Y resources to fix this one thing.

Taking on technical debt is a smart thing in early stages because it's essentially low-cost, non-dilutive financing. So you should recognize that very real benefit.

However, if they are young startups they might not even realize they are doing this smart thing. You can add value by telling them are smart and giving them the schedule of technical interest payments they will have to pay one way or the other if they are still in business.