But that's the point. It is a fuzzy thing. Software is built on business priorities, not technical ones. These align a decent amount of the time. Spending the extra time and money to set up tests is very much worth it.

Is it worth implementing a cache layer in a system? One on hand, you have a better system that can handle more load, on the other, you've spent time that could be spent doing something else. Your systems will almost always have limitations, it's important to understand these and communicate them well. Tech debt should be managed so it doesn't tip over and cause issues. Really good devs and dev managers develop the eye to see when that's about to happen and make sure they convince product that you need to pay that down soon.