Basically for mobile computers ethernet does not make much sense anymore in the grand scheme of things.
If you buy a laptop it's because you need portability, and generally that means wireless networks rather than wired.
Yes, desktop computers still have ethernet ports because it still makes sense, since they aren't moving around. And would logically be connected to a wired connection.
Outside of that, most current laptops from most manufacturers no longer have builtin in ethernet ports either. The ethernet connection is a bulky one, and removing it form laptops allows for sleeker, lighter laptops. And as the vast majority of laptop users don't need it or use it, it makes sense to remove it.
It's not that ethernet is being phased away, it's just being used where it makes the most sense.
If you require an ethernet port, there are literally thousands of USB-C to ethernet adapters out there, it does not absolutely need to be the Belkin one. https://www.amazon.com/s?k=usb-c+to+ethernet+small&ref=nb_sb_noss_2
As for the printer you can connect the printer to whatever router your Mac is connected to, not entirely sure why it needs to be a direct connection to the Mac over ethernet. The Printer should be able to be plugged in to your wireless router, and be found by the Mac then over the network.
It's not the Mac that needs an ethernet connection, its the Printer.