LAN and WiFi used simultaneously on Mojave

Contrary to what has been said, it is NOT normal for Wifi to lose connection if you have Ethernet plugged in.

Your system will only use ONE connection at a time, and that is to be determined by the "Service Order" set in System Preferences->Network.

Set the Service Order so that Ethernet comes first.

You should have to do NOTHING AT ALL in order to use Ethernet when connected and Wifi otherwise. Just connect and disconnect the cable, that's all.

(Alas, your wish to somehow balance the use of the interfaces based on speed or load is not an option. There is no way for the system to use Wifi to "help out" if Ethernet is congested, or something. )

Since you have a MacBook and sometimes use your laptop at home and sometimes elsewhere (obviously) you should set up a Home network location profile if you always prefer Ethernet at home, and an Away (we'll call it) network location profile.

Doing this, you can have your Home profile have no Wi-Fi (you can delete it from the service list) and only use Ethernet, and can do the opposite for your Away profile.

How to use network locations on your Mac - Apple Support

I am not sure why it is behaving like it is for you, but by having multiple location profiles you should be able to force it to act appropriately.

Sounds like normal behaviour.

How do you think it would decide which to use when given a “choice” of two live connections. It cant use both at once.

Maybe you never noticed it before. Back in the day when I had ethernet the LAN always took precedence over wifi.

I can understand that my friend wanting the flexibility of using Wi-Fi or Ethernet at home.

You cannot use them at the same time to get onto the same network. You would only really do that if you had two separate networks (like an internal "lab" wired home network + your ISP access to the Internet). That is called a multi-homed setup, but having two connections to the same network would create reliability issues and unforeseen problems. Also it would not give you better performance. You would want one or the other. Unless you have a very poor wi-fi network connection, you should be fine on wi-fi all the time. I am thinking one of these "unforeseen" problems is the fact you're hooked up to your wired ethernet and that's why your wi-fi is being occasionally grayed out.

In your case since you are both at your desk and in your terrace (or other rooms of your home) I would simply delete the LAN from your network services list. (or relegate it to a secondary location profile as a backup) Or, simply not plug into your LAN and you should be fine as well with the Ethernet device just sitting there, but I would set it lower in the priority list and make your Wi-Fi at the top.

“...the difference is that Mojave makes this visible by providing the visual clue of switching WiFi 'off', while earlier systems did not do that. Which is good to know...”

That hasnt changed to my knowledge and I started on OS Lion.

That is not contrary to what has been said it just confirms and adds to it.