The 5700X should easily overclock to 5800X levels while costing £280 at the MSRP while the 5800X3D will cost 50% more yet only provide 15% extra gaming performance according to AMD but remember that 15% is conditional on using a 3090 or 6900XT at 1080p res. Notch the res to 1440p or 4K or if your using a lower spec'd card and it essentially becomes equal.
At 4K I will notice a huge difference in turn time in Civ 6. Right now turn times for large maps late game can be around 60s. The 5800X3D is probably going to drop that in half if not more. At 4K in Stellaris I will notice that that 'fastest' simulation speed end game has not dropped below start of game 'normal' speed because the simulation has overwhelmed the CPU. Same in Old World with turn times too. Same will apply in town with Path of Exile where I will notice the lack of stutters due to my CPU being overwhelmed despite my GPU sitting at 8ms frame times.
Your statement is only true for AAA games. It is not true for a huge plethora of other games where the CPU matters more. Is there any point in going above 60FPS in Civ 6? No not really and even 30fps is fine for civ 6 or Old World because they are turn based. Is there any reason to go above 60fps in Stellaris, also no because it is a menu driven grand strategy game where you might zoom into a space battle to watch lasers go pew pew for the fun of it.
You also forget that over a 5 year span I am likely to upgrade the GPU and as soon as this year or early next it is entirely possible that whatever marketing name AMD gives Navi 33 will match 6900XT performance in 1080p and by RDNA 4 / Hopper that could be your £250-300 GPU.
It is also unlikely my gaming habits will change and the next round of Grand Strategy games or 4X games are very likely to be better threaded and CPU performance requirements will only increase. As long as Civ 7 keeps the strategy map even my current APU would be enough to run it at a comfortable frame rate, turn times will be the issue.
Another factor is that even though I have a 4k screen I play a lot of games in 1080p windowed mode because it is a 40" 4k screen so a 1080p window functionally equivalent to a 20" 1080p screen and I can shove other stuff on the rest of the screen like how to guides or path of building builds or even a damn movie if I want.
So I will go for the 5800X3D. I could have gone ADL but lack of testing in the games I play and means I don't know if they are cache or architecture bound so I don't know if they would be better than Zen 3 or not. The one game that does get tested is Civ 6 turn time and from what I can tell the 5800X matches the 12900K for turn time give or take. In that game specifically it seems it likes 8c/16t but it does not like the multi CCD chips as much so it seems that 5800X fastest, 5950 2nd, 5900 3rd and 5600 4th from the Zen 3 stack. Can't find any 5700G results (will check Gamers Nexus in a minute because I am curious) to see if the reduction of cache impacts that parts performance.
EDIT: I found 1 test for Civ 6 turn time for the 5700G and it was actually faster than the 5800X. That was OG Civ 6 though with 6s turn times not gathering storm where a good turn time is in the 25-30s range for 12900K/5800X. Unfortunately GN didn't test that game in their 5700G review so no idea how it performs with Gathering Storm.
Wish CPU reviews actually tested CPU bound games rather than this AAA @4k nonsense because it is a right PITA to plan a build for these games with a lack of data.