Imagine a bug like this in ext4. No one would reasonably contend that the layers on top of it should be validating that the files you write out are the ones you’ll read back in. We write unit tests for all kinds of stuff, but we’re not that thorough.
Then, subsystem B trusts reading the database field (since it passed validation layer A).
Obviously more validation layers can be added, but at this point validation layer C called by subsystem B needs to know what the initial input from layer A is in order to differentiate it from the db value which was manipulated - a rather tricky thing to do sometimes. (I guess you could add a hash to the db to check the db is storing your strings, but really.. come on)
Upgrading to utf8mb4 is probably safer than hoping enough validation layers thrown at it solves the problem.
But I’ve also seen people relying on validation at the time of insertion so many times that I wanted to warn against that, too. Not in argument against the need for utf8mb4.