You are not wrong about WiFi. But sometimes worse is better. :-)

With WiFi, I don't need any other technology stack, no hubs, no bridges. I can use my existing network infrastructure. The data packets from the ESP8266 devices get delivered directly to the MQTT broker over TCP/IP over WiFi. It's cheap (~$3 for a D1 Mini), dead simple, and good enough.

Every 15 minutes seems about right for temperature/humidity sensors. Increasing it beyond 15-30 minutes becomes a diminishing return, because at that point, the Deep Sleep power usage becomes greater than active WiFi usage.