Have you tried explaining it in a different way to them?
Imagine you brought your car for an oil change and the mechanic says "uhh yeah maybe you can get new oil next year but for the next few months we need to... Uh... Address technical debt in your car... So umm... It's gonna cost you thousands of dollars and you aren't going to get anything you perceive as a change"
Sadly most programmers are introverted and not well-articulated (which is much worse than being introverted). So when it comes to them you are quite right.
However, as a counterpoint to yours, even those of us who can communicate quite clearly get met with very ugly accusations of ripping customers off. People simply come to you with a preconceived notion about how much something must cost (in terms of money, effort and time) and they are absolutely not willing to change that notion -- even if they are on the side of the pure imagination and you are on the side of the facts.
It's basic psychology that many of us had to learn even if we never wanted to.
Imagine you brought your car for an oil change and the mechanic says "uhh yeah maybe you can get new oil next year but for the next few months we need to... Uh... Address technical debt in your car... So umm... It's gonna cost you thousands of dollars and you aren't going to get anything you perceive as a change"
You'd think they are ripping you off.