You don't need DDR5 on the 12700K either, just put a good 3600 CL16 kit and maybe tweak to 3800-4000 (that's close to the DR limit anyway).
It takes a very low latency DDR5 kit to beat that.
The 5800X3D is one heck of a way to end AM4 with a bang, excellent upgrade path for anyone running Zen1/Zen2 CPUs or people wanting to buy AMD in particular. It also puts to shame the 12900KS, there's a somewhat sour taste to be had for the gaming enthusiast who buys the Intel flagship, pairs it with the best memory kit money can buy, and may or may not get the best performance in all titles. That being said, it does little to change the strong value proposition of 12700K. Get a decent 3600+ memory kit, put a proper 125-150W power limit in place like a sane person, and obtain arguably better application performance & very close (if not identical) gaming performance. It's arguably a better all-round CPU, just like the 5800X3D is arguably the better gaming CPU.
In a way, AMD was shrewd to call the 5800X3D the fastest gaming CPU. This set the online conversation on the path they wanted, a comparison with the 12900K alone. Even if they "lose" the aggregated score by a few percent, it's still a win:
- value comparison leans heavily in favor of AMD
- 12900K wins on a technicality when every premium buyer wants the vigorous knockout
If I had to chose today between 12700K and 5800X3D I would be very conflicted.