That is definitely true. Databases are necessary. In fact in-memory data stores that can handle large volumes of data are not all that useful since they usually lack things like backups, etc. Not everyone is writing a RabbitMQ-like system. And of course locking plays a central role in all of this.

What I am saying is that when you accept that synchronization is going to be handled by your database of choice, it becomes somewhat less important how you actually structure your application in terms of performance. There are reasons not to use callbacks, but if you go with threads, actors, processes, etc. is now a choice between how you want to utilize memory and to an extent which technology your runtime supports best.