The ability to effectively diff and version control a LaTeX (or Markdown, or reStructuredText, etc) file is, more than anything else, what keeps me away from word processors for anything but throwaway document sketches. Word and similar can do "track changes", but it is extremely clunky by comparison.

You are using Word wrong (like everybody else). Don't manually change the font sizes on anything, instead give the text the correct style (title, subtitle, paragraph, etc) and then change that style to be what you want.

One of the benefits of LaTeX is that it practically forces you to do this.

When the easy way to do something is the wrong way to do it, it indicates a problem in the tool itself.

Edit: Somehow I missed your second sentence, and so we are in agreement. LaTeX is better in this regard, because it forces good practice on the user.

What if the user wants to write a letter?

I fail to see the problem:

\documentclass[a4paper,12pt]{letter} \signature{the user} \address{the user\\right here} \begin{letter}{Receiver\\somewhere over the rainbow} Bla bla bla ...

Not very different and you can ensure your letters look great.

Word: If you work with styles, changes should be rather easy and fast.