People buy games once their prices go down because the vast majority of them aren't worth $60 in the first place. I'll buy 2, maybe 3 $60 games a year (always at launch, if I'm not going to buy it at launch I'd rather wait that pay full price), and I'll buy about 5-10 20-$30 games. Unless I'm absolutely positive I'll play a game to death, there's no reason to buy it at launch since I could instead buy two cheaper games of equivalent quality. By next year, when it still isn't any worse or better than a new game, I can pick it up at half price. Just because price goes down after a year or two doesn't mean quality of new games goes up in that time. In fact, thats almost never the case. There are generally maybe 3-10 "revolutionary" or "must have" games released a year across all genres, and that usually leaves something like 1-5 that any individual actually wants. Anything outside of one's "comfort zone" of sequels and preferred/clearly defined genres can easily be put off until next year.
If you want to sell new games at $40, fine by me, they'll sell better and a lot less people will wait for the price to drop. But if you try to sell "part" of a game new at $40, then charge something like $60 for the rest, not only will you lose launch sales (because those that buy at launch are more interested than those who wait and likely want to play the full game), you'll kill off the lower-price sales too, as a lower launch price will undoubtably mean the game will remain at that price for a longer time, so by the time it's affordable there is either something better or the game will "only" cost what a new game does today, once you factor in the cost of the DLC.
DLC as a whole is effectively just a price-hike for customers, as you may be getting more content out of one game, but the developers time could just as well have been invested in the development of another game, and if release schedules all remained the same, everything would have more content, and it wouldn't cost the developers anything more. (With the exception of developers not planning any games in the future, but if that's the case they are probably more worried about bankruptcy than making DLC.) It's true DLC takes less time to make than a new game does, but if you start a new game now instead of making DLC for an old one, you'll finish earlier and have time to finish the "DLC" before the launch date.