You've already been swayed, but keeping these devices off the Internet was my main reasoning too. Sometimes WiFi is your only option, of course, but when the choice is there I go Zigbee every time. Other factors to consider include:
Zigbee was designed specifically for IoT stuff. WiFi was not.
lower power consumption
self healing mesh
much easier/cheaper to extend the range by strategically placing bulbs or smart sockets compared to extending WiFi range
avoids clogging up WiFi bandwidth with IoT noise
there's a limit to the number of devices most WiFi routers can handle
I assume Z-wave is similar but Zigbee devices are much easier to find, at least in the UK, and - someone correct me if I'm wrong - but I believe Thread is essentially Zigbee 2.0.