I see no reason to upgrade from the 2500K, if you are a 1080p player just OC it to 4,5Ghz and you are good to go, or change it to 2600K, today's games will benefit from the 2600K, and it is still a much better buy then any other new CPU.
If you are a 1440p or 4K player, then there is no reason to change the CPU, because bottlenech only ocures if you play on high end GPU and 1080p, so the CPU can't handle the fps. In case of 4K and 1440p there is bigger stress on the GPU, because of the less fps there is no bottleneck.
That's why I bought a new 1440p monitor instead of an upgrade from my 2600K.
You should only consider an upgrade if you are playing on 1080p AND 144Hz freesync monitor.
Other than that a new CPU is just a waste of money.
you explain it very odd but you are right for the most part. all except the very beginning.
if he is playing on 1080p and upgrades to high end vega the 2500k will 100% bottleneck the gpu. i would guess even a 2600k would bottleneck if he went high enough.
also a new cpu is not a waste of money if you are on the 2500k. a new cpu/motherboard/ram would give many new features, speed, reliability, cooler, and energy efficiency.