Zigbee devices are a pretty small minority of the larger IoT landscape. Most consumers are likely to have more wifi or bluetooth only devices than zigbee (at this point in time). The notable exception being Hue bulbs.

Apple is pretty much the only company in the smart home space right now that not only allows, but requires that devices be able to function locally, without having to call home. They CAN call home, but they continue to work just fine locally if say, the internet is down. It’s a central tenant of their homekit standard.