Using it for very-low traffic applications is simply not what it was designed to do
Was it not? Sure, it wasn't specifically designed for low traffic, but it was designed with a wide range of traffic levels in mind; after all, virtually every WiFi capable device, home automation or not, uses very little traffic most of the time as it idles. Now, I absolutely appreciate Zigbee or ZWave being optimized for minimal energy consumption, which is useful for some device types; but I feel it isn't right to call WiFi a poor choice for low traffic just because it also handles very high traffic well.
You raise an interesting point about congestion, though, and that is very much was I hoped to learn about. I am sometimes under impression that device designers assume everyone lives in a detached house so interference can be ignored. Do you know how specifically is Zigbee better in this regard? Living in a condominium I always had very poor experience with Zigbee reliability, which might or might not have been due to local radio noise at various ares of the spectrum, so I'm curious to learn about details how exactly these purpose-specific transfer layers deal with noisy neighborhoods.