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The desire for backwards compatibility/easier upgrades is why utf8mb4 could not just replace utf8:

The sorting order for the utf8 collations is not identical to utf8mb4 ones, and so an upgrade would require indexed columns to be rebuilt (remember, indexes are sorted).

I think with hindsight it would have been better to rip the bandaid off. 8.0 took a middle of the road approach by switching the default to utf8mb4.

They have. It's the default in MySQL 8.

Sorry, should have bern more clear: why didn't they fix the bug in utf8 so that it is correct (and would do what utf8mb4 does now)? It's just that the obvious choice of 'utf8' has been plain wrong for a long time.

from the article:

>Once MySQL published this invalid character set, it could never fix it: that would force every user to rebuild every database.

This can't silently be changed as the setting has impact on disk format for data and indexes and so and will be confusing when using different server versions, i.e. in replication.