You know how films, TV, music and literature all co-exist side by side with a large crossover audience... well who's to say all forms of gaming can't co-exist side by side the same way, they all work better for different occasions, sure ya can't take your monolithic PS3 with you but who wants to slump on the couch after a long day and spend a few hours on their smartphone?
The way we consume the different forms of gaming will constantly change and evolve, there's no doubt about that, but to say one will emerge triumphant over all others is deluded at best. The casual/mobile gaming market is just a different way of doing what we enjoy, gaming, and as long as they keep pushing and improving on that area it's a win for everyone. Even if you don't enjoy them many others do and they serve to drive innovation in all areas, the last thing Sony, Microsoft or Nintendo want is to stagnate and be eclipsed by more people having fun with their mobile games than their expensive consoles.
Competition drives innovation, the way I see it, the future is bright for all markets, but it probably won't look quite like it does right now.