There is no reason to upgrade.
I upgraded to an i7 7700K on the 10th of this month - only because my overclocked motherboard [i5 3570K] was dying after nearly 5 years of use. If it had not been for that I would not have upgraded.
You will get a performance jump in games that are CPU dependent or demanding, for GPU demanding games you will not see as much improvement.
I spent about $1,000 for an i7 7700K, motherboard, 32GB Dominator DDR4 3200 CAS 15 RAM, and Windows 10 Pro. For me, that was a huge investment for a little better performance.