The problem with this approach for me is that, not being a CEO, much of my email requires additional action other than simply replying to the email. E.g. Please review this 30 page contract or proposal.
Once I start down that route, the emails start to pile up, and tasks arriving via email need to be managed more effectively.
People this with problem I feel like are avoiding the core problem. You can schedule this task and put it on your calendar and let the person know. Or if you don't have time, let them know. If you are consistently over-allocated you need to make choices to free up your time, ask for help, hire people, reduce scope of work, etc.
Once I start down that route, the emails start to pile up, and tasks arriving via email need to be managed more effectively.