Latex is "typographically-complete". Markdown and friends are explicitly not. HTML+CSS is. But what latex has is a reasonable enough syntax that a human can write it by hand, unlike HTML+CSS. Moreover, the syntax, though clunky [1] is designed, as much as possible, to not interfere with the content that the human is writing.
For instance, Latex uses curly brackets {} for macro arguments, because they are least used brackets for content. So when you are reading a latex source, you know that () and [] are content, and only {} are ambiguous [2]. Nota, uses a mix of all three brackets for its syntax, causing additional pain for the person reading/writing the source.
The replacement for TeX/latex is never going to a simpler language. It is going to a language just as complex as latex. But it can definitely be cleaned up and sped up compared to latex. IMHO, somebody should write tex from scratch, improve it's syntax but otherwise keep it largely unchanged. Basically, any plain latex source using some of the popular packages should continue to compile and give the same output. That is the only reasonable way out.
[1] A typographically-complete language will never have a non-clunky syntax.
[2] Escaped brackets \{1,2,3 \} are literal curly brackets. Personally, I only use them for mathematical sets and have defined a macro \set, so in my documents {} are 99% not ambiguous.