I think when you start pulling on that string you eventually end up at event sourcing because once you are keeping comprehensive immutable history of all your changes keeping the mutable record anywhere just starts to look like a liability.

Yes, and it is incredibly comfortable to work in a properly tooled event-sourced codebase.

> just a hack to save some memory

.. and time. If we needed to compute an account balance by summing all the debits and credits since the account was opened...

Persistent data structures already solved this issue.

You don't need mutability to save on time, you just need to use referential transparency effectively when calculating your projections.