It's not so much being able to view or modify code that's the problem, but the fact that any modifications to an excel spreadsheet are inherently running in production. Open source has many benefits, but while open source projects may accept pull requests there is very few which put contributions straight into the latest build without review.

That's not a realistic comparison. I have yet to see one user try to modify Firefox's code. Spreadsheets, on the other hand ...

You are still making a distinction between user and developer. I think the point is that we can all be both.

That's not the issue at all. The issue is that these systems invariably have no review process, no testing, and no versioning (at best they'll have each spreadsheet versioned with no real way to know what each version contains). And this makes them incredibly fragile.