I think most people are used to seeing the source and can basically "imagine" what it looks like. So they really only switch for double checking before sending something out I would say.

Many people in our community said things about "staying in the editor" so I guess that's a common workflow.

Writting and browsing are often different mental modes. You are not always working with your notes, but mostly reading them. Thus have a plain edit-mode used 10% the time, and a rich viewing-mode for the remaining 90% of the usage-time is good enough for many people.

For me, I just don’t look at the preview at all. Markdown is simple enough that I know how it will be rendered.

To be fair, I do enjoy the Typora model where my images get automatically included in the edit mode, but I don’t specifically need it.

I honestly don't think people have ever used good WYSIWYG editors.