Some games already can use over 4 cores just not many. In 5 years who knows but I suspect a top end i5 will start to struggle if you want to max out games, but I think it would still be OK for high settings if you overclock it. Like a 2007 core 2 duo is today. If you got a 6 core sandy bridge E it may last an extra year before it struggles which I don't think really justifies the price.
I have a core 2 duo thats why i am upgrading.. So i am taking your answer with a grain of salt