If you really have that much data and traffic, the $ costs start to add up to multiple engineer comp costs. At that point it’s cheaper to move to something you have good control over.
I.e sharding at application layer and connecting to the DB instance replica where the customer data is hosted.
Depends. The cost may pay for itself but the engineers you have already may have higher ROI things to do. It's also nice to have operational stuff managed for you. Personally I'd be happy to pay extra for the kinds of problems Spanner solves to free myself up to do other things (to a point, ofc).
> sharding at application layer and connecting to the DB instance replica where the customer data is hosted.
Spanner does global consistency/replication. If having good performance per-tenant globally is a concern, this helps a lot, and is hard to implement on your own. It can also ultimately save you money by limiting cross-region traffic.
I.e sharding at application layer and connecting to the DB instance replica where the customer data is hosted.