That would be optimizing the database to be more efficient, and I am sure it's been thought of or considered. Available time might be a factor why it has not happened yet - or compatibility across versions of MySQL. You are correct, though - cascading deletes across table relationships is what grown up databases do. However, another good reason to NOT do this is the accidental removal of one innocent record could "cascade" throughout your database and delete things you didn't mean to delete.
Not sure of the advantages. Do you honestly think 20k extra data is going to somehow lag your database any? Buy a real machine.
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