Okay let's not fetishize this whole "complete at launch" "old school games didn't have DLC" thing. The problems with some games approach to DLC does not mean the whole concept is bad and, while they may not have been downloaded back in the day, expansions to released games were a common occurrence.
DLC/expansions when done well is a great thing for both the consumer and the developer/publisher. Look at Skyrim, by the time Skyrim came around Bethesda had settled on a good level for it's DLC. Each of Skyrims DLC releases was substantial and sensibly priced for the content. I'm sure that each provided a healthy profit for Bethesda and renewed enjoyment for the customer.
The internet has made this sort of post release extending of game content much easier and more cost effective, not making use of it when a game does well is as daft as overusing it on a game that's yet to prove itself. Just do it properly, with substantial DLC expansions that meaningfully add to the content of the game. Singleplayer/coop campaigns are a great way to do this and the assets developed for them can be added in free updates to the main game help keep the multiplayer fresh.
