I won't buy a game I know I'll never finish. I absolutely despise not being able to finish an otherwise good game. It ruins it. I'm not sure why you think it's fine to just dismiss other people's motivations for wanting to play a game.
In a sense, aren't you dismissing other people's motivations for wanting to play a game?
Sure the general population "can't" complete Cosmic Ocean (and I have "can't" in quotes because in my experience, there is very little in the world of gaming that truly can't be done by the average person with some dedication). But what about those who can, and/or are motivated by the challenge? Why do you consider it "good design" to dismiss their motivations to satisfy your pickiness about finishing all your games?