Google Cast and Airplay are very different and I have no idea why anyone thinks one would "replace" the other. I seriously doubt you will see Google Cast on receivers or other hardware for example. The Chromecast chief advantage is that it is cheap which you pay for by Google monitoring what you do with it and selling that information to advertisers. No thanks. It is also an inferior streaming solution being of lower quality partly because of the wifi implementation it uses which does not do 5ghz and partly because inferior hardware quality is the price you pay for being cheap. I expect this will be fixed in future versions but Google isn't selling Chromecast to make money from the device itself they are selling them to make money by monitoring what you do with it and selling that data. Just like all their other products. You are in fact the product not your Google device.
Personally I use both an Apple TV and a Roku 3. I can afford a little extra over Chromecast for higher quality video and sound, better connectivity, and far more content and features and not be monitored by Google.
I guess this is directed at me. For the second time, I never said Google Cast would replace AirPlay. I said I expect it to become dominant. Android is the dominant mobile OS worldwide, but it's not going to replace iOS, so I'm not sure why my statement on Google Cast is so controversial. I don't see how AirPlay can be the dominant streaming protocol in the future when the AirPlay experience on the dominant mobile OS (Android) is unofficial and poor.
If you live in a mixed IOS and Android household, AirPlay is not the best choice today (if you just want a single streaming option in the house), and it's not going to be the best choice in the future.
As I've said earlier, Chromecast is not the be all and end all of Google Cast. Regarding your doubt about Google Cast on other receivers - I'm not sure why you doubt this. Sony, LG, and Denon have said they will soon be selling speakers with Google Cast built in, and Sony & Sharp have said they will soon by selling TVs with it built in.