Excel already has versioned file formats with significantly different affordances, and hasn't appeared to have all sorts of compatibility headaches as a result.
Integrating a whole separate language ecosystem is one or two orders of magnitude more complex though. It’s more like HTML5/WHATWG suddenly adding Python as a first-class scripting language besides JavaScript, and web browsers having to integrate that.
It’s different because WASM is much more strictly defined and has a more limited scope than Python. Thanks to WASM, browsers don’t have be able to understand Python/PyScript. The trade-off is that you can’t, for example, view/edit the Python code in the browser’s devtools.
A WASM-like approach wouldn’t work very well for Excel, because you’re supposed to be able to edit the source code/formulas within the Excel application.
