Last year, I was looking to upgrade my desktop. Found a review that was bemoaning a motherboard because it only had two M.2 slots. How many consumers use two, let alone would benefit from a third?
I have opposite bemoaning for recent motherboards: only one PCIe slot (x16) is directly connected to CPU (not via chipset). It's useless to have PCIe 5.0 x16 slot even for average consumer because 4.0 x8 is still enough for modern GPU for gaming and even if it become not enough, 5.0 x8 should be enough for foreseeable future. Lack of high bandwidth dedicated PCIe slots for other than GPU makes the PC less expandable, e.g. video capture, another GPU (not for SLI), 10GbE, HBA, etc...
I hate it too. The allocation of PCIe lanes is garbage unless you spend $500 on a motherboard. We should have had the lanes divided better once we hit 4.0 speeds. At 5.0 speeds, it's absurd that 16 full-speed lanes would go to a single slot except for very specialist scenarios.
Part of the problem is that it's quite difficult to get a PCIe 5.0 signal to travel further than the first slot while keeping the motherboard price reasonable by consumer standards.
I suspect that AMD & Intel push the motherboard manufactures in that direction. You can certainly get more high-speed PCIe slots if you get one of the higher-end product lines (Threadripper, Xeon, etc.)
Yeah sadly now. PCIe x8/x8 or x8/x4/x4 (bifurcation) from CPU was common in standard priced board like Z170X-UD3, but now requires mid-high priced board. More flexible slots (dynamic switch by PLX chip) is always for highends.
Since the only thing consumers use full size PCIe slots for now is for a single GPU, extra M.2 slots isn't that big of an ask. One M.2 keyed for an interchangeable wifi card, one for NVMe storage, and then an extra one if you want to upgrade to more storage later. If you only have one keyed for storage, you're SoL when you buy a bigger one later and it's very inconvenient to move data over.
PC motherboard marketing and reviews usually don't count the WiFi card slot when tallying up the number of M.2 slots (since approximately zero motherboards are sold with an empty WiFi-type M.2 slot), so the complaint was most likely about not having more than two storage-type M.2 slots.
It's not a big ask, but it's also quite unimportant. Adapter cards to put one M.2 card into a PCIe slot are in the $5 range, and better adapters will do four.