Having said that I guess I broadly agree with your comment. It seems like a lot of people like to plan for massive scale while they have a handful of actual users.
In the first case, the database created as many problems as it solved (which is true of any large application running at scale; your data store will _always_ be suboptimal). A fancy, expensive NoSQL database won't save you from solving hard engineering problems. At smaller scales (on the order of tens-hundreds of RPS), it's hard to go wrong with any established SQL (or open source NoSQL if that floats your boat) database, and IMO Postgres is the most stable and best bang for your engineering buck feature wise.
Using Spanner is giving up a lot for the scalability, and if you ever reach the scale where a single node DB doesn't make sense anymore, I don't know if Spanner is still the answer, let alone Spanner with your old design still intact. For one, Postgres has scaling options like Citus. Or maybe you don't need a scalable DB even at scale, cause you shard at a higher layer instead.