I think that's much more of a technical argument than a practical one. Take a response like 'really?' in response to something. In non-tonal languages you can express a bunch of different connotations there depending on how you say really. In tonal languages you'd often end up saying entirely different words by doing such, so they often rely on things like particles to add intent/emotion, particles which simply do not exist in non-tonal languages.

Obviously you're right that the entire broad pitch of a sentence can also add meaning, but this is all increasingly alien to any sort of non-tonal speaker, and similarly the way "we" speak is alien to tonal speakers. It's not just learning new words and grammar, but learning a completely different way of expressing oneself and having to unlearn your old way of doing it. Hence why people who grow up one way or the other tend to do poorly with languages on the other side.

For instance a non-tonal speaker will find it extremely difficult to break the habit of trying to make a rising tone when asking a question, yet obviously in a tonal language you simply cannot do that or you risk using a different word. So they use particles at the end of sentences in indicate a question. Come to think of it, I should have made 'mama scolds the horse' into a question, as ma is also the particle to indicate you're asking a question! Now we're up to 5 ma's - ma ma ma ma ma.