A thing doesn't have to achieve a shoot-the-moon ending in order to be a non-failure.
(And in the particular case of the USPS, it's literally enshrined in the constitution because it was seen as a pillar of democracy to be able to securely communicate with others over long distances.)
This is not true in the US, the post office budget is separate and not subsidized by taxes.
But that depends on what counts as "success". If you are looking at just money or Github stars (or whatever the equivalent is in art or music) then that's one measure, but success could mean learning, personal satisfaction, something that helped or inspired someone else, the praise of a few people you look up to, or a job opportunity.
Almost everything new is hard.