It's taken as fact because economists have studied such things on the broad scale. If there was only one gaming console (and therefore a monopoly on video game platforms), there would be overall less rapid improvements. The company or companies controlling it would only make it better at a very gradual pace to keep people interested, which wouldn't be hard if there was only one place to go for video games and your only other options were other forms of media like movies, TV, books, etcetera. Granted, if there was still PC gaming there would still be some competition, but as it stands, PCs are always ahead in hardware compared to console anyway, so there would likely be more PC-only titles and Console-only titles as the gap between the PC and console would, if anything, widen.