Comment by hinkley

I think read replicas disagree with that pretty strongly.

The write traffic may be very write heavy, but then you have many, many users who need to see that data. The question is whether the database or a copy of the data from the database is what services that interest.

If you mediate all reads through a cache, then you have split the source of truth from the system of record. And then the read traffic on the system of record is a much lower ratio.