Email doesn't die because gmail can send to hotmail and to every other domain.
The entrepreneurs trying to replace it will focus their effort on one exclusive platform which they know they could monetize if they hit a tipping point. Unfortunately, the very motivation to profit is what creates the biggest problem.
The only true solution to email is a decentralized system, that many other companies could utilize. Unfortunately, once you have a system which is easy to create competitors you get instant commoditization. Once you have that, the economics of the idea suck which means that VC, and other investment money will never chase that system. In other words, a decentralized system is worthless from the idea stage due to the expectation of commoditization.
That is why email is “unlikely” to be replaced.