"Doesn't matter anymore though - they don't need a conviction."
Yes they do.
"They have your DNA and will keep it forever"
Wrong again, they are compelled to remove it after a set duration and then if asked, they have to by law. If it is found that they have lied, they can be done for contempt.
"so a little laboratory mistake down the road and you are a convicted rapist/child abuser."
Again, they can be sued for every penny leaving them no resources to police anymore. The police already have a battered reputation, this would finish them off, especially if you're a big name celebrity.
"The record that you were arrested gets reported everytime you need to apply for permission to work in schools, volunteer with the "vulnerable" or coach a kids soccer team."
Arrests don't typically get kept 'forever' and even then, they will be grateful not to see an actual conviction come from it, the judge doesn't say "You are free to leave this court without a stain on your character" for no reason. If they still use it against you, they can be sued for defamation of character and other offenses and you'd win.
"You will have to go through a long and complicated visa procedure to visit many countries - even if arrested but not convicted."
citation badly needed.