It's DLC that we're talking about here, not the game license itself. As DLC is only digital with no other possibility available to purchase that would mean every game out there with DLC will be in danger. Sure, a company can go under and all support will likely stop, but that's a different matter. Software licensing is an extremely complex area with other factors such as DLC playing a part in the whole debacle, different rules apply.
Whilst I haven't worked in software licensing for a good few years, so can't say for definite that this is the case, I can't imaging that everybody's DLC will no longer be available if Sony/MS/Steam or whichever distributor decides to shut the game servers down and stop support: there would have been an outcry already from that 'great gaming public' out there - let's face it, they get vocal at the slightest of things already, so I think this would be already a known thing.
I will look into this later, but as of right now, I think you are wrong with this.