Bethesda is not just a publisher they are also a developer. I also do know what publisher is and what they do, but my point is that this is different. It's not a complete reverse or anything, but it's hard to compare BRINK! to their other games, when all of them were built upon a series, and at least most of them they were working with id Software before. This is their first original game and they may decide to support it more because of that fact.
I'm not saying that you're 100% wrong I'm just saying that in my opinion there's a good chance they'll support the game with additional content. It would make sense and seems highly plausible.
Beth is a developer of other games, but in this case, they are only a publisher. They are not directly involved in the development of the game, so they will not be directly involved with any development after release.
SD has made games for other IP's before, but the gameplay was made from scratch each time, only borrowing the "world" of the series (Mostly storyline and visual style). In fact, the objective and class based gameplay is largely similar to their past games. The only time when the fact that they are creating a new IP makes a big difference is in creating the virtual world that Brink occurs in, which admittedly is a pretty big step.
Both SD and Beth are committed to continued support of all their games, so if anything, the fact that Beth is involved should be a push in the right direction when it comes to new content. All games developed by both Beth and SD have included DLCs or new content, at least that I can think of. This obviously doesn't include games made before the time when downloaded patches were common. (Such as Daggerfall)
So, as you can see, I agree that SD is likely to release new content, but my earlier point was that SD has been very careful to say that they will not be thinking about DLCs until after release.