I once heard this in a speech:
Hakaras Hatov is not about repaying someone for the good they did. Hakarah means recognition – you’re recognizing the good. Why is this the term that is used? Because sometimes, there is no way to repay someone.
I heard this at a Bonei Olam event, from one of their success stories. He said there was no way he could ever repay Bonei Olam for what they did for him. All he could do was recognize the good. He’d never be able to repay it.