Despite all the talk, there isn’t a world of difference between how apps used to be written 10 years ago and more modern frameworks. The tooling and the language have evolved, but you still manage state in similar ways, fetch data, build components, render.

Mainly you’ll be managing the rendering lifecycle manually in Backbone, which you’d end up doing in React anyway to get the best performance for an app like this - you’ll get molasses if you naively build a spreadsheet in react.

One thing I consider a huge advantage: you can read and understand the source in one afternoon. Makes debugging and optimization a lot easier.