Quad channel really isn't the point of HEDT, you have more than twice the pci-e lanes enabling you to actually use the expansion on your board instead of being limited to 1 16x and 1 4x which the later is generally cut in half when you enable some sata ports and god forbid you buy a high end motherboard which has a bunch of other things integrated into it which use pci-e lanes. You have ECC support(I assume), larger cpu choices, more robust components, power delivery, ect. too.
This means when you want to buy a few nvme's 4-5 years down the road as games expand to 200gb. Maybe slap in a spare video card for extra displays or vm's along with a couple usb cards for usb4, usb69 and whatever future things that say your Fractal Define R99 is going to have for I/O. There won't be a well, pick one or buy a new computer choice you have to make just because you wanted a new case and wanted to use your sata and nvme at the same time.
I'd recommend HEDT to people who are looking for long term platforms, someone who isn't worried about having 11/10th's single core performance and having the latest cpus. I've been running X58 for over 10 years now, which is an 11 year old platform pretty much at this point. It's seen like 4 gpu upgrades and a couple cpu upgrades, and I boot from nvme and have a working front USB3, which as far as I'm concerned my board doesn't support anyways. I have a GTX 1080 and get significantly limited by the GTX 1080 in 1440p on RDR2.
HEDT also out of the box runs more conservatively, overclocking is generally expected for gaming. As far as a stock 6800k to a 3900x. The 3900x is better in every way unless you're specifically looking for memory bandwidth or pci-e lanes. The pci-e 4.0 boards seem to be able to split them down into 3.0 lanes though so it's probably in the realm of what Intel HEDT users would likely use.