Explain that an in memory store is available.
Point out that it makes no sense whatsoever to use it in production.
An in-memory store for state is useful for a number of things: Running locally and testing (especially end-to-end or functional testing) for example. It is, however not useful in an actual production service.
The InMemory store does not persist your data between invokations - it only emulates persistence by keeping the events temporarily in RAM. When the executable terminates, your data is gone.
This is basically never what you want in production. So why have it?
As we hinted above, InMemory is valuable for running locally. There is no need do so some things, and it puts far less load on your local system.