The RX 580 is basically an RX 480 which was released two years ago now.
I've not got a crystal ball, but I think that in 2 years time, a card such as that will probably require you to turn settings down to a mix of Medium/High settings if you want to get near to 60fps. If only to keep within the 4GB vram limit.
One way to ease any concerns for the future would be to get a FreeSync monitor. That will allow you to play smoothly more than anything else. Some of the newer/better monitors have a FreeSync range going as low as 30Hz (think 30fps) for example.
Then there are the even newer 'FreeSync 2' compatible monitors, these use LFC to avoid stutter and tearing at <30fps.
That was the route I went down (getting a FreeSync monitor) so...