I don't think there's anything wrong with þ either. It fell out of favor, so it went away. If ß falls out of favor one day that's fine, but at the moment it's part of the language and that's fine too.
Thorn wasn't in the printing presses (imported from Germany) so the letter wasn't used. "Y" was in the font, but wasn't much used in English, so for a while that was used instead. "Ye Olde Shop"