LaTeX isn't the only way you can "write once"; while there are LaTeX exporters for the formats mentioned, you can do the same with LibreOffice, which exports to Word, PDF, and HTML much easier than LaTeX docs do (LaTeX conversion usually requires three different tools, it's a simple Save As... or Export in LibreOffice).
> LaTeX conversion usually requires three different tools
That's what shell scripts are for. It is trivial to write a script that will accept a LaTeX filename and run the necessary commands on it. It is only a little bit more work to add in hooks (e.g., to your IDE) such that the script is run automatically upon saving the file.
Moreover, with .tex files being plain-text, you could even write simple scripts to assist with updating the documents themselves. Something like "add-new-reference" or "update-hobbies" could accept input from the user and add it to the .tex file quite trivially.
This is in contrast to a GUI program, where you are basically forced to fire up a heavyweight editor, perform the changes by hand, and then perform each export one-by-one, also by hand. I know you could script old versions of OpenOffice using Java or Python, but that ends up being way more convoluted than doing the equivalent with a .tex file.
