It so happens that toms recently tested this exact question (how much performance do you lose pairing a modern card with an older gpu, compared to the same card with a modern cpu). Full results at [0] but the short answer is that a $900 RTX 4080 gpu with a 2017 cpu will generally do 60+fps at 1440p in most games, but as low as 55 in a few.
Not super surprising for single player games since they're usually much easier on the CPU than multiplayer. I was not getting minimum 60 FPS in Warzone for example.
I game on archaic i5-4460 (4 cores, max 3.2ghz), paired with rtx 2070 super and 16gb I can run literally everything new in 50-100 fps range, coupled with 34" 1440p VRR display not much more to desire for single player. Running this maybe 6 years with only graphic card and corresponding psu change.
Ie Cyberpunk 2077 everything max apart from rtx stuff and its rarely below 80fps, dlss on good looks. Baldurs gate 3 slightly less but still smooth. God of war same. Usually first think I install is some HD texture pack, so far never any performance hit to speak of. Literally why upgrade?
Consoles and their weak performance are effectively throttling PC games last few years (and decade before), not much reason to invest into beefy expensive power hungry noisy setup for most gaming, just to have few extra shiny surface reflections?
Comfortably doesn't mean everything will be at 60fps, it means that most things will be at 60fps so someone with a 7700k will not be feeling pressure to change their CPU (the entire point of the thread).
On the contrary I can say first hand that it is true (3700X which is two years newer, but on benchamrks it is a toss up between the two). What modern GPU at you using?
3700X's 8-cores is a generational leap above the 7700k's quad core for modern games. They aren't comparable at all.
I ran a GTX 1080 and then an RTX 3080. The performance was not very good in modern games designed for current gen consoles like pretty much any BR game for example. Some games got high FPS at times but with low minimum FPS.
I can say by first hand this is not true for any modern MP game.
In general, I hate these "it does X FPS at Y resolution" claims. They're all so reductive and usually provably false with at least a few examples.