Source? They said in the WWDC session (http://asciiwwdc.com/2015/sessions/706) that the process to disable rootless may change during the beta, but didn't say that it won't be possible in the GM.
They know that rootless will break some applications/drivers, plus some types of development may need it disabled.
They know that rootless will break some applications/drivers, plus some types of development may need it disabled.