Not saying they're not useful for specific purposes. But anyone buying them hoping to improve performance compared to their WiFi, often comes away very disappointed.
In my case A) and B) are irrelevant because I only really own or deal with laptops now days, and they invariably have built in WiFi, but usually not built-in Ethernet!
Your point makes no sense to me. A cable is often useful when WiFi isn't.
Case (A) is common for laptops. I've had plenty of WiFi modules (M.2?) go intermittent connection on friend's Windows laptops over time (maybe component drift?). For Linux on laptops I usually replace the manufacturers WiFi module so I get something better supported (high reliability - used to be Intel). Some people upgrade their module e.g. to get higher spec WiFi.
For (B), configuring WiFi routers is often easier with an Ethernet cable and sometimes necessary (depending on circumstances), and you need a cable to configure many other devices e.g. point-to-point links or whatever.
The fact you have a WiFi laptop is exactly why an adapter is really useful.
In my case, if I want ethernet it's because I want faster performance (reliably/continuously high bandwidth, and reduced latency and jitter) than my WiFi network can provide. But I've only been able to get that with a thunderbolt-connected ethernet adapter. Every USB one I've tried has been a disappointment.
I don't disagree that the uses you describe make them helpful in those circumstances, but I can't recall ever needing to do any of that myself. I'm happy with the built-in Wifi adapter and its drivers, and all modern routers can be configured/set up over WiFi, can't they? They create a default network when first turned on, or if you factory-reset them using the physical reset button.
I have a 2.5 GB/s USB to ethernet adapter. While I cannot say whether the performance matches that of built-in ethernet, transfer rates are fairly close to 2.5 GB/s. That is certainly faster than WiFi.
Oddly enough, point (A) is likely more relevant in the current world of laptops. At least if you use Windows. Plugging in a supported network adapter, may that be WiFi or Ethernet, may be the only way to get through the installation process, without jumping through hurdles, then install drivers for the built-in WiFi adapter, without jumping through another set of hurdles. (I own such a laptop, though I use Linux on said laptop so the WiFi just works.)