Monolith is fine if you have a fairly simple process for manipulating your data. Like posting an article, then a comment, with some moderation thrown in.
But when you start adding business rules, and start transforming your data and moving it around, then your monolith will become too complex and often too expensive to run. Lots of moving parts tightly coupled together, long-running transaction wrapping multiple joined tables etc. Rolling out new features will become very challenging as well.
Something like event sourcing is more complex to set upfront than a monolith, but at least it offers the way to add scale and features later without creating a combinatorial explosion...
But when you start adding business rules, and start transforming your data and moving it around, then your monolith will become too complex and often too expensive to run. Lots of moving parts tightly coupled together, long-running transaction wrapping multiple joined tables etc. Rolling out new features will become very challenging as well.
Something like event sourcing is more complex to set upfront than a monolith, but at least it offers the way to add scale and features later without creating a combinatorial explosion...