The difference between email and essentially almost any other "platform" (such as Zoho Connect, Yammer, Facebook, SMS, etc) is that email is completely independent. You can run your own email servers, you can configure it in the way you want, you don't have to worry about others at all. Not only that, since the protocols are open, unless someone has a malconfigured email server, or doesn't have one at all, you can email them even if there's never been communication between your networks before. You cannot do this with anything else and also be independent.
That's why email will not die. It may change in small ways over time, but it will still be here. It's the universal fail over for everything else (even some providers allow text messages and voice mails to fail over/forward to email), and I can send text, images, and all types of files through it. What else meets all of these qualifications? Nothing.
The only way to really get rid of it would be to make something just as simple, just as open, just as independent, and make it reverse compatible with email -- and then we're back to the beginning, Google's tried it, doesn't work out that great.
I also cannot predict what business communication will be like in 10 - 20 years, but I guarantee email will be there, just as a technologist in the 19th century wrote "I don't know what London will look like in the 21st century, but I know it won't be that much different than now." And he was right, because the fundamentals really don't change.
The same goes for email, and unified messaging won't replace it, considering that unified messaging almost always, without exception, centers around or fails over to email.
Saying otherwise is literally no different to me than people who have said over the last 5 years that "[mobile] apps will make the web obsolete within 5 - 10 years." and in fact are still saying that. I guess it's "disruptive" to say something will be obsolete, but with many, many, many offices still using software made 15 - 20, sometimes 30 years ago, it's more than a stretch to suggest everyone will suddenly shift to one direction over the next decade.