The R9 3900X is NOT in the same price tier as the i9-10900K... The R9 is $410 atm WITH a cooler, whereas the i9-10900K will be over $500 (significantly so if the i9-9900K was anything to go by; the $488 price on Intel's slides is only for a tray of 1000), and with no cooler. The R9 3900X is going to be at LEAST 25-30% cheaper in total (you're gonna need quite the cooler for that i9). As far as actual, real-world pricing goes, the i9-10900K is going to fall right into the no-man's land in-between the 3900X and 3950X (I'd be expecting $520-550 for the chip and $100 for a good enough cooler, so like $630ish total).
This CPU is barely beating a much cheaper CPU in gaming (at least at 1440p), while losing everywhere else, and with significantly worse power & thermals to boot (reviewer says a 360mm² AIO is the minimum you need). Not a good look for Intel.