Comment by 8fingerlouie

But if your one source of truth is a single huge database, maybe on a mainframe, as is often the case with finance, and eventual consistency is a huge no go, the mainframe can become a limit regardless when you're being "hammered" by 3+ million users all wanting to check their account balance.

Yes, you can solve the problem with sharding and other tricks, but for many banks, the mainframe is still their main data storage, and it has 60+ years of legacy code on it that is not easily or quickly migrated to modern architecture.