What I don't know that I've ever seen in these kind of "replace email" articles is an investigation of why, if email is so unsuited to this task, it is so widely used this way. It may turn out that the things the author thinks are disadvantages are closely related to advantages of email. For example, the complaint that, with email, anyone can "add something to your to-do list"; sure, that's a problem, but the fact that I can just ask any person I run into to email me a reminder without having to fiddle with my email system to allow them to do that, is actually a big advantage. The challenge for an email replacement, then, would be to allow more control over this without introducing any more friction.