I think you're talking past each other. If you want to build a web-based spreadsheet application, you neither "progressively enhance" nor do you "gracefully degrade". Your browser is either a supported runtime or it's not.
Sure, if you're building a content site, you will want to have progressive enhancement, but if you're building a client-side application, SEO concerns and the other concerns of content-delivery sites are less relevant.
Sure, if you're building a content site, you will want to have progressive enhancement, but if you're building a client-side application, SEO concerns and the other concerns of content-delivery sites are less relevant.