This was announced a year ago, developers have had plenty of time to port over their extensions. If they haven't done it by now, turning off legacy support is really the only thing that will make them do it.
I know I'm not going to bother porting a fully working product to a new API unless I absolutely have to and I'm sure most other developers would agree with me. Although I would have at least tried to sync up the availability of my new extension with Firefox dropping legacy extension support, but you can't always make those deadlines.
Biting the hand that feeds IT
