Keeping DNA permanently from people who commit of non-violent minor offences is excessive. There is no reason to think anyone might go on to commit the type of crime where DNA evidence might be relevant. The action of taking DNA in this situation unnecessarily penalises people who make a once-in-a-lifetime error, minor enough to be dealt with by a police caution.
I think it is wrong that somebody with Simple Caution should be retained on the database for life.
Remember, people who have been cautioned have not been arrested, have not been charged, have not been to court, and do not have a criminal record.