"In an ideal world, no one would have to be "forced" to behave like a decent human being because they would already be one."

But the World is not ideal, and can't be made that way. Attempts to create an ideal world are littered with millions of dead, cruelty, and misery. Communism, or whatever you want to call the collectivist scheme you assume will replace our lives of plenty, doesn't work because people are not altruistic and can't be made that way. Even the religious have the hope of heaven in everything they do.

"Unfortunately, we're surrounded by a vast number of selfish assholes..."

Absolutely! So tough guy, are you going to kill them or lock them up and brainwash them? Neither? Okay so they are not going away then are they? Any system that is designed that can't accomodate people like that will fail. See: USSR, etc.

"libertarianism is a godawful mess"

Do you mean the Libertarian Party? Fuck them. I don't see why a real libertarian would be in a party anymore than an anarchist would. Freedom means freedom from being in a mess, and also from paying dues, going to meetings, or being responsible to a bunch of random knuckleheads. Libertarianism is a way of seeing the World without any illusions, and still believing that humans are good enough that they don't need their hand held by some power. I can live and see the world and behave like a libertarian in the world the way it is now, but a socialist can't live like one until everyone else gets in line and lives like a socialist too. Ain't gonna happen.