* people typing markdown in text files, where you want to split paragraphs into word-wrapped lines of a sensible length, and consecutive non-blank lines form paragraphs just as they always have in text files;
* and people typing markdown into text entry boxes on web pages, where you would like pressing the <enter> key to actually mean something.
These two situations probably prefer a different default.
Then make the form that the user enters data into be either parsed differently, by pre-processing it, or have some javascript magic that automatically adds the extra two spaces required to make it work.
This way if the user presses <Enter><Enter> then the JS can remove the extra two spaces and it will still be valid markdown.
* people typing markdown in text files, where you want to split paragraphs into word-wrapped lines of a sensible length, and consecutive non-blank lines form paragraphs just as they always have in text files;
* and people typing markdown into text entry boxes on web pages, where you would like pressing the <enter> key to actually mean something.
These two situations probably prefer a different default.