I see it as the opposite. If you give a 2 panel markdown editor to any non-developer human they will be baffled at how to do any formatting with it. Rich text editors are intuitive, that's why they've dominated computing for the last 30-40 years.
> you can't easily predict what will happen simply by looking at the screen
Huh? In rich text you see what everybody else will see. In 2-panel markdown you need the second panel to see how your final text will look to the rest of the world because you can't predict that easily when writing in an esoteric markup.
> For this huge advantage in predictability and simplicity, Markdown sacrifices formatting power
I wouldn't call requiring a second panel to preview how the actual text will look predictable or simple. It's a rube goldberg machine. On the other hand, I think the best thing about markdown is the restrictions to formatting. It forces you to write in semantic HTML.
IMO, Notion, for all its sluggishness, nails this part. They provide the intuitiveness of a rich text editor, while restricting the amount of styling to a markdown-esque set of properties so what you write will be easily portable to the web.
Both tools do it differently. Typora shows it when your cursor is there, but otherwise hides the Markdown. Bear always shows the Markdown but applies the corresponding style to it.
Rich wysiwyg editors also seems to have issues where the visible editing and internal formatting state breaks or behaves in weird ways. Like when you insert links and it automatically formats it as a link in some cases, and not in others.
Edit: Personally I always though the split view was okay as long as the compile times weren't too bad. Having a markup editor and then a view of the result is what I do for coding, so why not when writing stuff as well.
The whole point of Markdown (in my opinion) is that the formatting is a function of text that you can see and edit. The entire state of the document is visible, and there are no formatting operations you have to learn, just regular text editing operations like backspace, copy, paste, and so on. For example, if you press backspace, you always know exactly what the result will be: it will delete the character to the left of the cursor.
This is not so in rich text editors; when you press backspace, it changes both the document and invisible internal state, and you can't easily predict what will happen simply by looking at the screen; you need to be aware of the invisible formatting structure, or perhaps a recent history of things you did, or it might be entirely impossible to tell because it depends on some invisible state that was created weeks ago and saved in the document file.
For this huge advantage in predictability and simplicity, Markdown sacrifices formatting power (you can only do a limited set of things like headings, lists, bold, and italic).
That's why Markdown tools that hide the markup are, in my opinion, the worst of both worlds: you sacrifice formatting power, and you sacrifice editor predictability. After the markup is gone, how do you edit it? How do you unbold something, or change the indentation of a list? If there are separate commands for all that, then there was no point in using Markdown.