It is unfortunately true that it is impossible to publish a partial work. A publisher cannot put half a novel up on the shelves at your local indie bookstore. The first duty of a writer is to finish writing the thing.
A fellow BVC denizen tells me of a writer she knows who was unable to meet deadlines. Even after signing the contract to deliver the novel on a given date! Publishing houses have schedules to meet. They have booked space/time at printers, and when the time comes, there has to be a manuscript ready to go, or a lot of money is lost. When this author blithely told the editor that the contracted ms would be six months to a year late, what did he/she expect would happen? The slot went to an author whose ms was finished and ready to go.
Another sad example might be the Golden Heart award handed out by the Romance Writers of America. The prize-winning ms is almost always published — if it is a complete work. In the past the prize was won by people who had their three chapters and an outline, which they had been grooming for ten years. When the author won, there was no novel — she had never learned to complete one. And it was too late to learn! It got so bad that the RWA changed the rules so that only complete mss can be submitted for the prize.
Does everyone remember Robert Heinlein’s rules of writing? To become a writer, write, he said. And finish what you write!