I disagree... Markdown is no longer a single unified version. There are multiple implementation, extensions, etc. The original Markdown spec by itself isn't that useful and there is a reason so many extensions have been made for it. It can maintain simplicity while still adding new features. Pandoc has some great examples of Markdown extensions that are really useful.

The core features are pretty much universal and used in a lot of apps (chats, forums, git repositories, project management). Custom flavors maybe not rendered correctly at times but this doesn't make documents unreadable

The core features used in a lot of apps include different flavors, like GitHub Markdown and others. The closest thing you have to a unified Markdown is CommonMark and even that is forked for GFM. Yes, headers, bold and italics text, and lists are nice but there is a lot more to take into account when you are trying to supplement docs that other extensions add.

There are dialects of Markdown but they all respect the core features.

true, but please notice that you can use asciidoc as a larger subset of md

Which is not something I want. If I wanted something more complex I would use TeX.