Typically, Ethernet uses a star topology. Which means that if you were to use a protocol based on TCP/IP —like MQTT— for wired devices, you would end up with a (number of) central node(s) and one cable for each device. That's a lot of cable.
I was researching alternatives for wired devices and the ones with best support or quality (in Europe) seem to be KNX and Loxone Tree, but they are proprietary and expensive. I'd much rather stick to open protocols.
Is it possible to have a bus topology for home automation using Ethernet? Ideally, I would throw one cable and connect devices (mainly smart actuators with ethernet connection) in serial, and be able to address them individually.
The idea is to complement a wireless installation of non-critical sensors and actuators which is using Zigbee and MQTT, with a wired installation of more critical devices, like dimmers, doors, blinds, etc.
Ethernet is already running through the system (for LAN or cameras), hence the question. A different approach is to have a bus installation that can "talk" to MQTT without needing extra gateways.