Photopea.com exceeds CS6 but keeps the UI. All sorts of plugins, great project. You can also use various hacks like photoshop UI for Gimp and things like that, but I found that route to be unusable.

I've converted over a dozen weird edge cases of spreadsheets and access apps and ancient scripts used by departments into standalone little apps or browser apps, ranging from budget and finance related bookkeeping to tracking sales to licensing management. The only advantage Excel has over this is ease of maintenance - it's a lot easier for someone to guide themselves through updating things on a spreadsheet, or to break an idea down into multiple pages, etc, if spreadsheets are what they're familiar with.

If you're an engineering or finance firm dependent on an obscure, unique Excel feature, I could at least see the argument that your use case is too hard to migrate off of Windows.