No real reason to get rid of the 3570k.. It is still quite a viable CPU.
The thing is, 1080p stresses out the CPU. The higher the resolution goes, the more stress gets put on the GPU.
For example, if you put a 1080ti in that, and run The Division at 1080p on Ultra your 3570k is maxxed, but your GPU is at 93% and its pumping out >130+fps.
Change that to 1440p, all of the sudden, your CPU isn't maxxed (altho its around 75-85%, but it isn't Pegged at 100% like 1080p) and the GPU is 98% (maxxed), but you're still pumping out 70+fps.
Go even further and run 4k.. Your CPU is all of the sudden 65%, GPU is 98% (maxxed) and you're doing 50+fps.
In my opinion, you don't need to replace it. It can run 2k/4k easily. Upgrade GPU and Monitor to a higher resolution.. 1070/1440p... 1080/4k...