And there's nothing about the nature of email that is asynchronous, gmail has blurred the lines between email and chat. An email reaches me just as quickly as iMessage et al
My guidelines are use iMessage/bbm/txt for conversational inquiries where there is a need for back and forth type interaction and email for everything else. There's no difference in their delivery time with gmail.
My daughter thinks of email as just for school stuff. Everything else is over other mediums, mainly iMessage (mainly due to the magic like "..." Indication) or Instagram. Time has changed things. Email is now as instant as instant messaging and IM is now as media rich as email, supporting photos and video etc. the lines between these have blurred mainly due to the smartphone. Once your messaging channel was coupled to the delivery vector, but now my MacBook Pro can receive iMessages and my phone can receive emails. I don't want a unified messaging interface because I can use different ones for different people and don't feel anxious about any of this. If I want to avoid the world for a while I have a mute button.