This thread reminds me of work (part of it anyway).
One of the things I end up doing involves Microsoft Word; automating production of documents that are at times 50+ pages tennes of thousands of times a day. Microsoft Word is probably one of the furthest things away from Markdown possible, but there's one problem I run into daily.
Half the embedded stylesheets that get used have 0 line spacing between paragraphs. The other half do. Basically none of them are consistent between 20+ years of legacy templates, and the end users disagree on preferences on how much spaces should even be between paragraphs. (Part of my current stuff is saying from a business efficient POV, disagree all you want, I'm going to make things consistent regardless because there's a ridiculous amount of problems caused by not being able to even know what something is going to look like until it's already rendered, at which point, it'll look broken most likely if there's not consistency in approach).
That is, your average desktop user is completely willing to just spam the enter key repeatedly to create space, ignoring the fact it's the equivalent in HTML to having <p></p><p></p><p>Actual text</p> and all the problems associated with algorithms that try to keep pseudo <h1>Title</h1><p>Text</p> logically connected on the same page which fall apart.
Word also has shift enter, which is a "line-break", and is treated differently by the layout systems. I see people realise this exists in Excel (where enter works completely differently) but even those users do not know the semantic difference in Word.
And ultimately, I'd imagine TinyMCE builds its defaults around users expectations, where the significant majority of users base their defaults on Microsoft Word due to that lovely proprietary stack in the education system, where one enter key is absolutely one logical paragraph regardless of whether it's visually lacking in space or isn't.
The analogy from Word to Markdown is clear in the specification, the Word equivalent of a line break in Markdown is simply a single carriage return to go down to the next line. The equivalent of a paragraph mark is two carriage returns in a row with nothing in-between them, a simple \r\n\r\n
IMO, it makes more sense for Joplin to have an option to override the renderer/CSS to remove line spacing between paragraphs than it does to flip the behaviour of the enter key to act as if it's a line break bind and not a paragraph bind, especially if you were to e.g., ever consider exporting your notes in alternate formats like HTML.
So in my opinion, the defaults make sense from a user experience point of view because it emulates the behaviour most people are familiar with. In the context of Joplin specifically with the markdown editor and the actual preview being separate, I can see it being annoying in the markdown preview; but from a technical correctness POV where the content and presentation of content are separate, in my opinion it makes sense to treat that as a styling/theme problem and not a content problem.
(Appreciate this isn't really an answer, but thought people might find it interesting anyway).
Devils Advocate mode: I personally feel markdown encourages this when I use it outside of Joplin (e.g., Nano):
Every sentence should be on it's own line.
This is still a paragraph.
If I see this. It's wrong. Git diff works better when these are broken up.
So even I'm a hypocrite, because swapping the enter key would make sense in a lot of contexts too, I write more sentences than paragraphs.
