How are they going to sell us their Hair DLC, and Horn DLC, and Armor DLC, if we can just make it ourselves?
Fan made mods has never impacted sale of actual studio made content.
The people that use mods are a subset of the market, and the mods will never have the advertising power of the publisher behind them.
Also, fact is that aside from minor changes generally story, new areas, and new missions are not done by modders. So this is mainly texture improvements that leverage the PC Platform, aesthetic changes, and fixes that the developers can't do because of budget or policy constraints.
Skyrim is totally moddable, and its DLC sold like hot bread.
Same with Neverwinter Nights.
Adding hair and horns to a DLC that includes story and game content I can see, but standalone cosmetic content for a game like DAI for SALE?
That I can't see them doing. Look at Shadow of Mordor DLC. Some of it was cosmetic, but always with something like a new game mode or set of missions bundled in.