Comment by wtallis
According to PCPartPicker, 2x48GB kits at DDR5-5200 are currently the cheapest DDR5 on a $/GB basis, so you really need to have a workload requiring more than 96GB of RAM to justify having more than two DIMM slots. That's pretty solidly into workstation territory.
Is it far off in terms of $/GB in a normal user starting with 2x24 GB 5200 at ~half the up front cost while still being able to upgrade 4 years later, if needed, when memory is even cheaper and RAM requirements are higher versus all that pro/con difference against being able to run 2 high end DIMMs at 6400 instead of 6000 or similar difference by limiting the number of slots on the board. Again, I still think this makes plenty of sense for boards focused on OC enthusiast but paying more for the 400 MHz on a tradeoff of not being able to expand just doesn't seem particularly worth it on normal boards for normal users.
Or, perhaps stated differently, other than enthusiasts purchasing the best binned low density RAM who else has what practical gains by general consumer boards having fewer slots?