Second, people often use analogies to illustrate. I could have used code (as in computer code) to illustrate the tension between complexity simplicity, but the point remains the same. I used the law because the law is written in language and specifies rules (like a game) and is written and interpreted. It's also a pretty good analogy. There will always be a conflict between simplicity and complexity. There is no "just create a base that answers all questions, and let people modify it." But if an analogy is not sufficient or does not carry weight with you, just use experience; think back to any edition of D&D, no matter how simple (yes, you can go back to the 70s) that did not spark lively discussions about rules interpretations.
And this is where it all boils down to game design. Yes, D&D has a lot of complicated elements in it, which need rules. This is common among all role playing systems: It tries to simulate combat, or tries to resolve things that would normally be handled by the computer in a video game, by using dice.
I went to Spiel in Germany this year, which may be one of the largest board game conventions in the world. And one of the wonderful things of attending this convention, is that you get to meet a lot of up and coming game designers, and try out their games. And you run into a ton of games that while very pretty on the cover, just have very complex and unintuitive rules, and way too many mechanics crammed into it.
But there are also games that manage to do a lot, with very little rules. That is the elegance of game design. You want the game to be very flexible, without inventing a new rule for every little detail. Add too much, and before you know it, your entire game is filled with clutter to represent every health point, mana point, money, item, special item, etc, etc, and the size of the manual rivals The Lord of the Rings.
I won't deny that D&D is a complex game by design. But it has slowly been getting more streamlined and coherent with each edition. I do believe there's such a thing as writing perfect rules, where there is no confusion or debate over them.