True, but just using Excel does not lead to decent data models, though. IMHO, what we would need is for "regular" folks to grasp the basics of relational database first (at least 1:n and n:1) and then build an easy to use excel-like tool around it.

DRY is mostly fine in code, but does not hold for data. There, inmutability and audit trails win over DRY, in my opinion.

A single source of truth is even more critical for data. It is the principle behind normalization.