> Like think about how 'That's a great idea.' can be phrased in different tones to make it sound enthusiastic, insulting, a question, sarcastic, and so forth. You just can't do that in tonal languages.

You certainly can. Just because a morpheme has a tone doesn't mean that there aren't other contributors to pitch, such as sentence pitch contour. Do you actually speak or understand any tonal languages? I can assure you that Chinese people definitely use sentence prosody to convey extremely subtle nuances of meaning.