I agree with some of this (i.e. the numbers), but I think it's coming from a place of not understanding what Markdown is for. For example, your complaint about indentation for code blocks makes no sense if you're using Markdown appropriately.

The original idea was to take emergent syntax used by text file authors as cues for humans reading text files, and use that as a syntax for generating HTML, so you could use text files as the source of truth for generating HTML.

Backticks aren't an emergent syntax used by text file authors as cues for humans reading text files--that's a syntax directly intended for generating code blocks in HTML, with little semantic value if the markdown is going to be consumed as text.

If your goal is to have something consumable as both text and HTML, then some sacrifices have to be made, because text simply can't do everything that HTML can. The error-prone indentation syntax is just a compromise, but there were features that were left out completely. The original markdown didn't contain underline or strikethrough, for example, because text can't do those things.

The problem is that for a few years, this idea turned out to be useful as a way of allowing users rich text editing capability on websites. So people wanted Markdown to do everything that HTML could do, hence we get underline, strikethrough, code fences, etc. This hurts use cases where markdown is used for its original purpose, because modern markdown isn't as nicely consumable as text files, but users didn't care because they weren't using Markdown to be consumable as text files. And this made sense for a while.

And where this becomes stupid is that if you don't care about it looking nice as text, then you shouldn't be using Markdown. There are a wealth tools for rich text editing on the web (TinyMCE being the most obvious choice, but not the only one available) and they're FAR superior in user-friendliness to Markdown. Many actually support Markdown, but there's less and less reason to even expose that functionality to your users. If you don't care about how your Markdown looks as text, then just use HTML and edit it with a rich text editor.

There's another use case where you want to compose HTML content without the overhead of actually writing HTML, but you want to "do it like a coder", so using a rich text editor isn't appropriate because you want to be able to diff and whatnot. But in that case, there's still no reason to be using markdown, because if you're "doing it like a coder" than you can just write code which makes what you're doing much clearer, and give you a lot more power. Textile, Org-Mode, LaTeX are all better options.

GitHub has persisted in using Markdown well past the advent of mature web text editors, because they still seem to be operating under the illusion that nontechnical users will become a significant part of their user base. Slack and Reddit are both moving toward fully rich text, and the fact that they are still trying to expose their Markdown roots just makes their rich text painful to use. MediaWiki using their markup language still, probably because they are blocked by the enormously difficult task of migrating an enormous amount of content, but if you're starting a new project, you don't have any of the tradeoffs that these organizations do, so you shouldn't make the mistake of following in their footsteps.