email doesn't need any replacement. it's a fantastic messaging protocol. It's used by millions and every system supports it. Maybe in the far future it will disappear as other messaging protocol will rise, but we still use phones and snail mail, and there are very old. What need to be replaces is the bridge between email and todo protocol. The task management tools, even now, are not solving the problem of intelligent task management and creating more work than reducing it. You have to file, categorize, make priorities, or even type the tasks yourself. That's the reason people don't use them and use email instead as a task management tool - which it doesn't support.