Instead of talking about D&D as one game, for the moment let's think of it as multiple games beneath one umbrella. I think this makes a little more sense in the old school world, which tends to love multiple subsystems and approaches for things. If we look at it that way, we can ask each individual element what its 'game' is, what makes it uniquely fun. What is the combat game? The adventuring game? The domain-building game?
So the question becomes: What is the alignment game?
First Edition has a couple of uses for alignment besides character behavior. It is used for various protective spells and magic items, and in the utterly bizarre idea of 'alignment languages'. It makes sense for multiple alignments to cooperate so as to accommodate many possibilities for languages, items and spells. Even though I believe the rule prohibit this to some degree (I don't think Paladins can knowingly hang out with Assassins and whatnot), any party would still benefit from possessing several alignments. It's possible to invent all kinds of similar plays off of this idea, from magic items that work differently with different alignments, to gates that only allow certain alignments to pass through unless disabled. So alignment becomes a benefit or a detraction based on circumstance, another choice that players have to make and then make the most of, like how many gallons of lamp oil or how much wolfsbane to pack.
This doesn't mean there aren't alternatives to the alignment system as presented. I just want to suggest that instead of attempting to come up with a better set of moral principles or roleplaying aids, let's try thinking along the lines of what sort of game we want to play with an alignment system. Then we can maybe get over the idea of alignment as a set of dubious restrictions and think of it as another facet of the complex world of adventuring.