It is covered in this Wikipedia article. It is called 'the incessant obsolescence postulate'. https://en.wikipedia.org/wiki/Interstellar_travel

"It has been argued that an interstellar mission that cannot be completed within 50 years should not be started at all. Instead, assuming that a civilization is still on an increasing curve of propulsion system velocity and not yet having reached the limit, the resources should be invested in designing a better propulsion system. This is because a slow spacecraft would probably be passed by another mission sent later with more advanced propulsion (the incessant obsolescence postulate)"