They definitely could play another 8 years, but I doubt they would be motivated for that long. Within a couple of years, there would be very few records left for them to break. They have achieved almost everything there is to achieve.
The only thing really left for them is the CYGS, which they could already have if they didn't choke at the end of the Slams so often. Including this year. More RG would certainly help their legacy. And the Masters Series titles they haven't won: Indian Wells, which is just bizarre that they haven't won, since they are from California. Montreal and Shanghai.