This kind of game becoming successful would have been impossible before Steam. In the old days the brick-and-mortar stores would refuse to stock any game that was even remotely controversial in content or age rating. Steam has been hands-off regarding what they allow outside of things that are illegal (or, recently, that their payment processors disapprove of - if you want to talk about influential monopolies that shouldn't exist...).
San Andreas and The Elder Scrolls: Oblivion were even pulled off of store shelves temporarily due to their age ratings being adjusted. Places like Walmart are a hundred times worse gatekeepers than Valve has proven to be.
Dark Souls as well.