Yep! This aligns with my understanding as well. Saw a trending tweet that likened Suspense to Error Boundaries for data fetching which I think sums it up nicely
If all the data is fetched before render, outside the render tree (like next.js) there would be no need for progressive loading when render occurs right?
It’s only when a fetch invoked during initial render, which throws a promise to trigger the suspense, that the loading staging is valuable. This seems to me to encourage the “waterfall” design you outlined – I can’t tell if this is good or bad though 
I can say that I see a lot of sites in the wild that could benefit immensely by consolidating the loading experience which I hope suspense helps