It depends on what is meant by in-memory database. The most useful kind IMO is the one which actually saves everything to disk, but is not designed like a traditional RDMS in that it assumes everything, including indices, can be saved in memory. Therefore, you don't need a complicated buffer pool system and you don't need to touch disk at all after startup to service read queries. The most simple approach to such a database is just to MMAP a file.

This kind of workload is probably the most common in all of software development for the past couple of decades given how plentiful RAM is as well as most applications having some need for storing persistent data.