lol. So much revisionist history, it's making my head spin!
Okay, basically PS1 sold roughly about as much as NES did in the US and Japan. Sony's real inroads into driving their userbase (and the console market in general) was in Europe, and there they largely cannibalized Europe's established and diverse computer gaming industry anyway (R.I.P. Amiga, Spectrum, Commodore, etc).
The truth of the matter is, Nintendo's always brought more legitimately new gamers into the industry than Sony has. They've traditionally done this by bringing in kids for the first time with Super Mario Bros. or Pokemon, but this generation saw them working from other ends and bringing in women, seniors and other traditional "non-gamer" segments. What Sony did (in the US/JP at least) was take a page from Sega's playbook, and upmarket their platform to the aging base Nintendo helped build in. In essence Sony's market was less "new" than it was courted and aging... most people came into gaming with Nintendo though, even if more people chose PlayStation in the late 90s.
"Mainstream" is really in the eye of the beholder. I'd say gaming pre-Wii/DS/iPhone was still largely seen as an expensive, infantile hobby for teenagers, social reclusives and manchilden. Adults still played Atari 2600 or the Famicom though too... I'd argue what Sony did was more continue the course and natural evolution of the industry (which itself, was naturally aging) rather than really making some dramatic and unique "first" in terms of legitimizing the medium within popular culture or to people not already indoctrinated as gamers.