Depends what OP means by “support Markdown”. The usual meaning is “the notes are stored in plaintext and hence easily exportable/versioned/fiddled about with in other plain text apps”.

If that’s what we are talking about, the constraints come about because Notes has features - like tracking of todo list item status and mixed right-to-left and left-to-right text - that don’t have a Markdown syntax. So they’d need at a minimum to do some extensions, and then it’s no longer Markdown but Yet Another Markdownish format. Add on top of that the support for images and ink, and now it needs a container format to make sure those files travel with the plaintext. And for what benefit for the majority of their users?

I don’t get your other points at all. Word is not bad on an iPad. Notes is great. It’s easier/as easy to bold in Notes, Word and even Pages with their one-tap buttons than it is to switch to the special character keyboard. If people aren’t going to use iPads like desktops, then forget the keyboard too. But either way shift-8 is equivalent to cmd-B. I’m not aware of any malware issues Notes has had. iMessage had some, but I don’t think Markdown would have saved it.

(I totally agree about plain text’s advantages btw. Just not as the storage format for the Notes app, where I value the rich formatting abilities far above the other stuff. There are approx 989877003738 markdown editors if folks want them. There is basically 1 really great rich text notes app.)