I was editing some markup yesterday (wikitext, but the principle is the same) and came across a document formatted like this:
Heading Paragraph Text Heading Paragraph Text
There are only single line breaks between headings and paragraphs. This demonstrates that there are existing documents are formatted is this way. If backward compatibility is a concern, it is dangerous to change the behaviour, even as an extension (which we should aim to degrade gracefully).
From the front page of commonmark.org, the description of CommonMark is (emphasis added):
A strongly specified, highly compatible implementation of Markdown.
And this:
We propose a standard, unambiguous syntax specification for Markdown, along with a suite of comprehensive tests to validate Markdown implementations against this specification. We believe this is necessary, even essential, for the future of Markdown.
For better or worse, this is a conservative goal, not a progressive one. To formalise Markdown, not progress the language in a way that breaks backward compatibility.