> You can easily buy two terabytes of memory nowadays. Hence, scaling memory is relatively easy (save the time it takes to make that data hot in memory). This means your first million customers are easily served from memory, and it'll take a while before you reach a million customers.

However, by this time your entire business depends on submillisecond responses from Mnesia in memory and you can't transition to a different database, or even to on-disk Mnesia.

This has happened :)