They didn't and will not put a kill switch in. That would make them vulnerable to lawsuits.
They can leave it behind, and let the community rip it apart, which is long overdue.
Imagine the REAL progress that can be attempted when you can leave having to write legacy code for a 12 year old OS behind.
I am not an Apple fan, but every 5 years or so that retire an OS fully.
time for MS to do the same.