That's a different question now. Before it was "do I not trust my switches and routers?". Now the question seems to be "do I trust all devices on my network?".
If you have guests in your network, surely they have a different level fo trust.
Anyway: If you trust _all_ devices on your network (including routers, PCs, phones, printers, light bulbs, air conditioners, thermostats, doorbells, scales, TVs, set-top boxes, coffee machines), and the software running on them to be bug-free, unrootkited unhackable, and upt-to-date, and their vendors to be perfectly honest and reliable, and those ones' employees, hardware, and software supply chain to be perfectly trustworthy, forever, then yes, you don't need any form of cryptography in your network.
Otherwise, you need something like TLS for it to be secure.