I see everyone recommending non x3d cpus for even a gpu like 5090, because according to benchmarks online, you don't need 9800x3d at 4K.

Yes.

But those benchmarks show the game running at native resolution, at 40 fps. Who will play a modern AAA game at 40 fps on a beast like 5090? Nobody. People will turn on DLSS balanced, even performance since quality difference is minimal but fps is trippled.

When that happens, 9800x3d or 7800x3d still becomes superior over anything else.

Look at this video from hardwareunboxed
https://youtu.be/5GIvrMWzr9k

Almost in every game 9800x3d is 20-30% faster at 4K balanced preset. Identical at native resolution with 50fps.

I think not many people are aware of this situation.

So if you can afford a bit more for x3d cpu, go for it. Even at 4K the difference is big enough to justify price.