The only way software is truly finished, is when customers stay the same forever.
Spoiler, this is impossible.
Once humans get accustomed to something they want better. Civilization and innovation are built upon that idea. Once the humans created the first car, a faster one was being built. Then a more stable one, eventually we are now even sending humans into space with space shuttles. Why? Cause humans always want better.
Software products are similar here. Once the first digital calculator was invented, we didn't stop there. We started to add more complex functionalities and calculations. Then it was ported to all the different platforms including your smartphone. Then there are various themes on how the digital calculator even looks.
Software is never finished, cause people's desires always evolve. To satisfy these desires software needs to be adjusted or even completely rewritten. This is not the nature of software, but human nature itself.
Software is thus never truly finished.