In my head? Half Life 2 was the beginning of the end for that era. Maybe not exactly at that point but it was a great game that brought even more people to steam than ever before. From there we now had a centralized marketplace for games with a massive audience and we start seeing the creeping corporate influence of "microtransactions", the death of the expansion and introduction of dlc story content, beloved franchises abandoning originality in the pursuit of mass market appeal. That kinda thing. Of course if not half life 2 then it would be the Elder Scrolls Oblivion, who truly started something horrible with their horse armor dlc.
The future is largely Indies or AA games, everything else is going the way of call of duty at this rate.