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 t

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.