Uh oh, that assumption is a big no no.

depends

Can you elaborate? Honestly asking.

I wouldn't recommend it with PCs, notebooks, phones, random crapware, but:

When you control¹ all the devices on the network, the network is small enough and the danger from the non-authenticated protocols isn't too high, then I would say it is reasonable to assume being present in the network is sufficient authentication. Not saying it could not be improved, but there are probably many more pressing concerns.

¹ you don't fully control anything anymore, but you're not going to fix that either.

The air gapped power plant networks I work on have unused Ethernet ports shut off and the ones in use only accept traffic from the MAC address of the device that is meant to be there. So you can’t just show up and plug in.

Thanks a lot. That gave some perspective. I'll keep that in mind.