The fundamental source of poor email style is the practice of quoting the entire message you’re replying to. If that’s what you do, then it doesn’t matter whether you put your response at the top or bottom. In fact, if you’re going to quote the entire message, top-posting probably is better. But both are poor form.
Writing an email is like writing an article. Only quote the relevant parts, interspersing your new remarks between the quoted passages. Don’t quote anything at all from the original message if you don’t have to.
Forwarding is not replying. If you’re forwarding an entire message, sure, add your own remarks at the top, as a preface. But the idea that each new reply in a thread ought to contain the entirety of each previous message in the thread is silly, wasteful, distracting, and unnecessary. The point of quoting isn’t to maintain a self-contained copy of the entire thread in each single message; the point is to provide context for your own remarks.
Does it take more time to edit the portions of quoted text included in your reply? Yes. So does spell-checking and proofreading. It also takes time to shower and brush your teeth each day.