Nothing is stopping you from authoring your math however you like as long as there are tools that support the translation[1]:

[1]: http://fred-wang.github.io/TeXZilla/

I don't think that LaTeX is a significant competitor of MathML, just like markdown isn't a competitor to HTML. If you're writing something for the web, you're highly unlikely to be directly inputting HTML, whether it's on Wikipedia, StackOverflow or Hacker News, so you'll need some sort of server-side processor or a javascript library previewer, anyway.

Native (browser) support of MathML is far more convenient for the reader and not more cumbersome for the writer.

I wish people used LaTeX.

I only get "what's that?" most of the time.