My generation (born in early 70's) was the first to really come up with access to video games pretty much as a given. We had space invaders etc in the arcades (or wherever else they could fit a cabinet), hand-held Nintendo and basic home consoles in primary school and then of course came the boom with PC, PS, X-Box and all that followed.
But a lot of people only a few years older than me are just as dismissive about games now as they were then - this is kids' stuff, a waste of time and money. I'm not an obsessive gamer, but I do play every now and then and I absolutely see games as just another option on the entertainment landscape.
And being that older people weren't into them, I guess I'd always had the thought in the back of my mind that games were something you grew out of, like binge drinking or not caring about your retirement (your examples may vary). But the other day I was playing with friends and I looked around at these ~50 year olds and realised we're never going to "grow out" of this - particularly if the hardware and software just keep improving the way they have.
So for me one of the big reasons that gaming will keep growing is because those older, non-gaming generations will die out, replaced by new generations who start gaming ever earlier while eventually you may find yourself playing call of duty with a platoon of 80+ year-olds.