Soldato
I've had a 4090 on a 5800x for over a year, it definitely holds in back expecially in rt games.
We seen this though when it was the first time in as long as I can remember that a GPU was clearly waiting for a monster cpu to compliment it back around release. All the fomo to get the best value yet it was going to be paired with weak components. Unfortunately there was no in between sku just a gulf down to the 4080 which was 1200 at the time. The 4070 was not any leap if you were on strong ampere/rdna2 to warrant the prices. So to compliment the beast you have also got to overhaul the other components making it a big investment rather than a drop in upgrade.
Not sure why anyone would be in a hurry to switch out the 4090 for a 5090 in most cases, you would be better poised moving off AM4 and seeing what a top fresh cpu build can offer the 4090..