I'm not sure I agree with that sandbox comment. I think (in the main) it's the complete opposite, personally.

If anything, it was the era of multiplayer. 4 controller ports effectively became standard. Xbox Live, despite not appealing to me, ushered in a new era of online gaming. It's easy and it works well. The PS2 came online eventually, to a mixed reception. And Nintendo were criticised for not taking the GameCube online. Downloadable content became the norm. And so on.

This era brought in a new philosophy: the multiplayer component is as important as the single player game. Fact.