It mostly has to do with turning down the Transfer Speeds per DIMM once you add in the 2nd DIMM onto the same channel.
Because you're adding extra electrical load onto the Memory Controller, your speed is compromised.
However, the question wasn't about adding a second dimm to the channel, it was about giving the
possibility to add a second dimm to the channel, i.e. having a socket for it. And yes, there is a super minor difference because of the added trace lengths, etc, it's not really until you actually add the dimm that there is a problem.
Doesn't really matter if it's AMD or Intel, their memory controllers Maximum Transfer speed will get noticeably NERFed if you add in 2DPC.
That's how it has always been, that's how it's going to be.
Show a benchmark, remember we're not talking about adding a second dimm here, we're talking about having the socket for it. We all know why a second dimm will nerf the speed.
I'd rather see companies moving to 1DPC and just give you 2x the Memory Controllers to compensate so you can get speed & capacity if that's where you want things to go.
The cost is prohibitive, but that is in a way what AMD has done going to 12 channels. I agree that that is a way to solve it, but it also means a LOT of more pins on the CPU, and traces that need to be laid out, as the 2DPC config is relatively easy in comparison. Some workloads would be better off for it, however, not all need the speed and will say DDR5-4000 or 4400 is good enough, they just need the amount of memory.