I’m on the data side at a water operator, so I see this argument from the people who want dashboards. Our sites run exactly this split and it’s held up for years.
The PLC does control: it scans I/O every few milliseconds whether the network is up or not, its inputs and outputs are made for 24 V field wiring and noisy cabinets, it has certified safety options, and the night-shift technician can open the ladder logic and see why a pump didn’t start. An ESP32 can do the logic, but you’d be building and certifying all of that yourself, then supporting it for 15 years.
Hermetry does monitoring and supervision: history, alarms, dashboards, operators sending a command. Even Hermetry’s own ready-made water solution keeps it that way, the pump stations‘ automatic start and stop stays in the station’s controller and the cloud only switches manual mode and pumps on command. See Water site. If the internet drops, a cloud rule can’t stop a conveyor.
Where ESP32s do make sense: extra sensors the PLC doesn’t have, like vibration on a motor, temperatures in a cabinet, energy meters. Cheap, no risk, and they go straight to Hermetry.
Kevin, the cost question is the one to answer with numbers: one hour of downtime on a line versus the price of an S7-1200.