Thanks for the response, it's nice to chat with you :)

latency of the medium is so equivalent as to be practically unmeasureable

More or less, yup. There are some cool uses of RF to achieve very high bandwidth, low latency connections (5G as a common example, but Wi-Fi 7 has a theoretical maximum speed of 46Gbps - while this is still far behind the maximum speed of Ethernet (We have 400Gbps Ethernet in use, with 800Gbps in development), it's catching up very fast - and since most households and businesses with copper cabling will be using mostly CAT5e or 6a Ethernet (1Gbps/100m and 10Gbps/100m respectively), Wi-Fi will soon likely be faster than most copper Ethernet networks. It's also very likely that 5G internet will all but supplant ADSL and VDSL connections in the coming years. I think twisted-pair copper cabling is following in the footsteps of coax :)

Even with in-home fiber

The minimum latency of a connection through fiber is about the same (actually, slightly less, but not enough to matter) than the same connection made through copper. Signal propagation speed is not a benefit of fiber over copper - the benefits of fiber are that you can have many, many more connections in the same diameter of cable than with copper, it's immune to electromagnetic interference, and it can run much further distances without needing signal boosting.

most WiFi routers don’t have particularly fast CPUs, or high-performance buses.

That's one of the main issues, yeah - consumer grade electronics are usually total junk, especially the free routers provided by ISPs, but I'm also thinking of those absolutely horrible "gaming" Wi-Fi routers provided by the likes of ASUS - they have decent specs, but they're just absolutely overloaded with features that gobble RAM and CPU. Dear consumer electronics manufacturers, please just let the router be a router, and let the Wi-Fi APs be Wi-Fi APs. Combine the router and the Wi-Fi AP if you must, but absolutely please stop suggesting that people can run a hundred services from routers. You should totally upsell that feature in a separate node appliance or something! Sorry, I got distracted.

it’s cheaper and easier to get a fast ethernet switch than a fast WiFi router

I agree, but I also don't - most consumers don't really know what a switch is or why they might need one. Most switches found in houses are either integrated with a router, power line adapter, or Wi-Fi access point. While a good switch is absolutely going to be much cheaper than a good Wi-Fi AP, most people wouldn't really look to buy one. They might search for "Ethernet hub" on Amazon and luck into buying a decent switch, but I think most people think in terms of Wi-Fi these days, so it's probably easier to get a Wi-Fi AP than a switch.

Also, just a minor nitpick: "fast Ethernet" is a little confusing, as terminology, because that's the marketing name used to refer to 100mbps Ethernet connections (often indicated on network devices as FE) - so named because it was the successor to 10mbps (regular) Ethernet. (damn you, marketing people! I blame y'all for what you did to USB) When we discuss this kind of thing, it's clearer to refer to 'high speed Ethernet' or refer specifically to line speed (e.g. 10GbE) - unless we're talking about 100mbps Ethernet! Although, even then, it's probably a bit confusing these days - I'd call it 10/100 Ethernet usually, rather than fast Ethernet, unless I was being really lazy ("yeah just stick it in the f/e port")

I doubt any of this has as much of a latency impact as WAN factors

It definitely can do, but in a properly functioning network, I'd agree. If you have a faulty connection or significant source of interference or impedance, then that would be much more of an issue than anything else - otherwise, yeah, it's going to be the Internet where most of the latency comes in to play. I would estimate that probably 75% of people could get big improvements to their online experience by making changes to their home network, but at a certain point, yes, contention becomes the bottleneck, which is not so easily solved :)