I wouldn't count on future Intel processors sticking with the same socket. I also wouldn't count on ASRock managing to get support for future processors right in their currently shipping motherboards.
On such an extremely tight budget, you'll have to go with some of the cheapest new parts available today. $400-$500 is an unworkable price point for a gaming PC. $600-700 is more likely, as you can see from several other recent threads
in this forum.
If you're serious about wanting a complete gaming PC for under $500, you might look for a scratched-and-dented Inspiron 620 or 570 in the Dell Outlet store
and then just add a $145 Radeon HD6850 or $170 Radeon HD6870 to it for gaming.
i7-6700K, NT06-Pro, GA-Z170N-Gaming5, 32 GiB, 980Ti, SM951, 5TB HDD, Blu-ray, FTZ01, SX600-G, 3007WFP+2001FP, RK-9000BR, MX518