DLC can be justified, just not all of it. DLC that has a good reason and a reasonable price can be justified. As can free DLC.
DLC that has no other reason to exists then to take money. This can be as in pre-order only DLC, skins with no additional basis to exist and over priced DLC with one or two minor items.
The fact that so many companies have taken this poor model on board is what causes this issue. If more companies did worth while DLC's then it would have been seen in much the same way as expansion packs, which are basically the same thing.
Yet companies have often decided to go down the path that has the highest monitory value, and with so many people desperate to buy everything for their games who can blame them.
Myself I am not keen on DLC, usually waiting for a "complete" edition of the game, or just waiting until everything is on a deep discount if I find there is going to be DLC's (there are exceptions such as SotS:TP). Although a lot of people complain about day-one DLC, I can see why it happens. For a lot of companies they have teams that finish a long time ahead of others during development and putting them to use is fine. The situation remains that the things these people work on should not be limited DLC for pre-orders only as this still dissuades customers down the line who may want a complete edition.
The same thing for me goes with Kickstarter. I hate (even when I want to back) Kickstarters which do the whole "exclusive in game content" crap. Why not just say we will create special DLC that you will get day one / when ready completely free, and all others will pay for later. Shadow Run Hong Kong did this for the game based extras, so I feel less guilty knowing others can buy the extras (except some unique in game portrates). Yet other games have entire characters, or even quest lines that only Kickstarter backers get and that annoys me.
A good DLC here on GOG is;
The Witcher 3 DLC pack.
http://www.gog.com/game/the_witcher_3_wild_hunt_free_dlc_program It is free, adds a fair amount of new and interesting activities to the base game as well as being optional.
A bad DLC here on GOG is;
Any pre-order only DLC.
So, Kyn (armour), Grand Ages (skins), Age of Wonders 3(map pack) and their ilk.