You can perfectly chat with emails, the problem are that all clients make you use emails as digital letters instead of chats. But you could perfectly imagine an email client doing what gchat used to do.
It's more a metter of perception. When you click on a mail, it takes a lot of time to type, then your UI have a delay to say it's sent, then you have this time taken by many clients to move your mail to the proper folder. But a proper mail-chat client would just make the message appear as sent, then to all the stuff in background, and you would just not see it slower than other chat. The time people take to answer you message is usually the bottleneck, not the network.
Users are extremely sensitive to latency. I would ditch a chat client with any real latency in a heartbeat.
https://play.google.com/store/apps/details?id=com.pingapp.ap...
Unfortunately, my colleagues insist upon using Skype instead of email, apart from the few I've persuaded to use Google hangouts instead.
Something secure, flexible, workable, and open needs to replace it.