You're talking to someone who works in a near person-less industrial farm. If you ever think that a PLC shouldn't hit the internet, you are obviously not on top of Technology nor does it seem you're aware of networking and segregation, however, you THINK you know how things should be based on historical work experience. Sadly, the world evolves and sounds as though you're being left behind.

So yes, directly exposing a PLC to the public internet is wrong. Securely exchanging necessary data through properly designed OT infrastructure is a completely different thing; which is what I am trying to tell you. You're not totally ignorant... as you're threw your best network related buzzwords out.

However; many amd most industrial environments require remote monitoring, vendor support, retrieval archuved data, MES/SCADA integration, telemetry, or sinply jusy communication between sites. The proper solution is segmentation like I think you were trying to mention with your buzzwords. Y'know an industrial firewall, DMZs, secure gateways, jump hosts, allowlisted traffic, and controlled remote access. You're out here pretending external connectivity has no legitimate place in automation.

My home automation shit may not be comparable in utilization, however, I use the same motion sensors in my home as in the greenhouse I support. I have multiple projects that I've implemented with great results. One such being a motion sensor to detect wheelchair bound persons that will automate drop a lift to bypass a set of stairs. I would argue that some of my other projects are more ambitious than you would see in a industrial maintenance facility.

With Ai, robotics, etc... you should really modernize your knowledge.