I think based on your experience that's a fair line to take; I've spent a lot of time on the ops side, and seen much of "well we did nothing and we're all out of ideas AND budget"

The best answer is somewhere between these two extremes, and I like your approach of starting simple and clear - a file based approach also easily lends itself to centralizing or bootstrapping to another medium - the bigtable approach calls for a migration and potentially even a change in the guarantees made by the underlying system because someone started to repurpose it as not just an audit trail because "of course we can just query it." (ugghgghghgh)