Bad, simple, and dumb are not mainstream.
I dearly wish that were true, but unfortunately a lot of media that finds mainstream success today is just that. People here often flirt with the idea that because something is popular it must be good.
Truth is, and this has been pointed out, the term "mainstream" has absolutely no impact on quality. As a Hollywood worker bee I like to make comparisons to the film industry--I know it's not 100% anologous to the video game industry, but I think in this case it's relevant. Transformers 2 is, I think, bad, simple and dumb. It made 400 million dollars, and at the screening I attended people cheered themselves hoarse. The Social Network was one of the most intelligent, clever, and moving movies I'd seen in years. It made much less money and when I saw it the theater was 3/4 empty. However,
both movies are as "mainstream" as they come. Made by giant studios with famous directors and famous actors and tons of money. It's not like the better movie is some obscure scrappy underdog that came out of nowhere and ignored the rules of what makes a film popular. Quite the contrary.
With that in mind, the TES series has
always been mainstream. I first discovered Daggerfall by reading a gushing review in PC gamer, one of the most mainstream PC magazines at the time. The issue had, if I recall, Command and Conquer (one of the most "mainstream" games of the era) on the cover, illustrating several tanks blowing up in glorious eye candy explosions. The popular myth that TES was somehow the domain of an elite cadre of hardcoe fantasy RPG nerds with 200 IQs is completely false.
So "mainstreaming" is not the problem. Mainstream products can range from godawful to sublime. It's a word used to talk around specific gameplay problems. However, I dislike being labeled as some sort of gaming elitist because I'm concerned with the way changes made in Oblivion will affect Skyrim. There were some
baffling gameplay changes in Oblivion that should give any TES fan pause. The complete lack of lore development, the overall blandness of the game world, the insane level scaling, and mechanics like insta-healing after an hour of rest and instant travel definitely constitute a trend in the wrong direction.
On the other hand, every "mainstream console kiddie" I know who came to the series with Oblivion has the same issues with the game as "hardcoe RPG lovers", so blaming the bigger audience for the simplification of the series is misguided.