If you practice not closing, you can do without any markdown language. As the HTML itself gets almost as simple as markdown. Plus, gives you advanced control not found in markdown.
<!DOCTYPE html>
<title>Example Webpage</title>
<h1>Welcome to my webpage</h1>
<p>This is some content!
I agree with your remark regarding the redundancy of markdown. It makes almost no difference to write markdown or html5 by hand. If people knew this, it would seem they would replace their "markdown engines" with cat (to put the header to each page), or simply with nothing at all.
Those are (rooted in) SGML elements having start-element tag omission allowed, whereas HTML's "self-closing tags" (in HTML5 parlance) correspond to elements having end-element tag omission allowed. In your example, SGML infers the missing tags for html, head, body, and p to arrive at the canonical form:
<html>
<head>
<title>Example Webpage</title>
</head>
<body>
<h1>Welcome to my webpage</h1>
<p>This is some content!</p>
</body>
</html>