Are LAN sockets still up-to-date? WLAN/wireless is the future!

It may be a crutch, but it works. Our entire administration building is connected to the network with Devolo. Works great. 5 offices, 1 meeting room, 6 PCs, and a notebook. All via Devolo.

I understand that. But the argument "no LAN socket = no TV via Telekom" does not hold. Anyone who really installs more than 1 or 2 double sockets in every room (except the living room) either has too much money or not much knowledge about technology. Even if in 10 or 20 years all kinds of devices are equipped with network connections again, you still don’t need that many sockets. But well, everyone should do as they please.

I have a question regarding this: When do you use duplex and then single? What is the advantage of a duplex cable? Is it absolutely necessary?

There is no advantage and it is basically nonsense, and the "easier" handling does not justify the extra cost. Two cables work just as well, you can pull them through together just the same.

I have a question regarding this: When do you use duplex and when single? What is the advantage of a duplex cable? Is it absolutely necessary?

Duplex costs the same as two single cables. If the conduit diameter is limited and there are several bends in the concrete ceiling, then the duplex is easier to pull through than two single cables.

you still don't need that many sockets

It's not about needing them all but rather that there is one present at important locations. I'd rather plan for 5 sockets and 10 outlets too many than one too few.

Maybe those things cost nothing for you, but I think we paid 100 euros for a double socket.

If you do it yourself, a socket might still cost you about 30€ including materials. I chased the walls myself, plastered myself, connected myself, measured myself. The only thing the electrician mostly did was pulling in the cables.