Is teaching like working on a product but leaving before it is released?

I think that's a somewhat synical way of thinking about it. I view teaching more as incubating an egg. You try to give good information and inspire your students to learn more, so that they can prosper. Sure, you might "never see what your students will ultimately achieve", but I wouldnt look at it in that light. Teaching isnt giving up on something before it's finished, it's trying to push them to be inspired or interested in learning more about the topic you're teaching.