Wait, are you saying you have a PG27UQ (a ~£1800 monitor that costs £1300 more than some of the best 1440p 144hz monitors on the market) but you play at sub-4k resolution and you intend to keep playing at sub-4k res even with a next gen GPU?
Playing at below native res on a monitor often looks worse than playing at native res on a lower res panel.
Assuming you intend to play at 4k resolution on your super expensive 4k monitor with a next gen GPU, then:
1) you are unlikely to see a difference moving from an i5 to anything else in nearly all current gen games.
What CPU is a good match for what GPU is not a constant, fixed value. It depends on the games, and on the resolution. The higher the resolution, the more GPU-bound you are, and the less
the CPU matters relative to the GPU.
At 4k with a 2080 ti:
The Ryzen 9 3900XT is the flagship of the new AMD Ryzen XT series. It comes with higher boost clocks and can sustain them better, which helps with single-threaded workloads. In our Ryzen 9 3900XT review, we also saw better overclocking and lower temperatures than on the original Ryzen 9 3900X.
There's almost no difference between anything. You're more or less at margin of error with a 9600k vs even a 10900k.
2) Obviously that's with a 2080 ti, and it could look a little different with a next gen GPU. However, only a little
Napkin maths: Even at 1440p, the differences between CPUs can be quite slim
. 1440p is about half as demanding as 4k. Given that next gen GPUs won't be twice as powerful as current gen GPUs, the differences between CPUs at 4k with a next gen card will still likely be closer to that they currently look like at 4k (i.e. very small)
3) Nevermind when next gen titles launch which are even more demanding, and we go back to being even more GPU limited within a few months on new titles.
There's certainly no point to buying a new CPU until you know for a fact you need one - rather than on the off chance you maybe might need one.
This would likely be throwing in good money after bad.
With an i7 or i9 from 9th Gen you're paying near as much as 10th Gen. You're going to see almost no real-world performance gains today. By the time you do need more performance from your CPU, far better stuff will be on the market. It already is (Intel 10th Gen) and there's a decent chance AMD's Zen 3 out in November could offer better performance than say a 9700k, for a lower price even factoring in a new mobo.
Keep what you have for as long as you can. Then replace it + mobo (+ RAM if we're on DDR5 by then) for something far more impactful.
Don't sink more cash into an obsolete platform.