Just like Free to Play and Pre-Orders, there is nothing inherently wrong with Downloadable Content. In fact, all three have some striking advantages when done properly.
Unfortunately we see too many companies taking advantage of these things to nickel and dime consumers as much as possible. That's what we should be speaking out against, not DLC in general.
The main practices I have a problem with are day one DLC where content was cut from the original game and sold separately, and DLC that negatively affects the experience for anyone who doesn't own it.
ARMA 3, for example, does DLC well. Many people argued that $16 for two helicopters was too much. As a flight sim enthusiast, I'm used to paying $50 for new DCS World modules or $15 for the older ones on sale. ARMA's choppers aren't nearly as detailed, but at $8 each that's not bad from my perspective.
But here's what makes it cool:
Your $16 for those helicopters directly helps fund the features that were built and given away to everyone for free when the DLC launched.
Oh, and the new choppers won't prevent you from playing with your friends who don't own them. They can still ride in the helicopter with you and sit anywhere, they just can't fly it (even that has a workaround in the mission editor by forcing the player to spawn in the helicopter as a pilot).
So your $16 gets you a couple of big birds, but at the same time it helps fund the development of all those awesome new gameplay elements that the entire community gets for free.
If you ask me, Bohemia is doing DLC right. There's no reason to take your rage out on companies like that. Focus on the ways companies are abusing the DLC model, not the the model itself.