They've already started moving away from using his voice.
Examples:
-The elevator announcement for Park st on the red line is done by someone with a snarky voice
-The countdown timer announcement uses the "Mike" voice from the AT&T text to speech software
-The delay announcements on the subway uses the "Crystal" voice from the AT&T text to speech software.
-North/South/Back Bay station announcements formerly used the "Rich" voice from the AT&T text to speech software, they now use "Crystal"
-The "See Something Say Something" announcement at all stations used to use Frank's voice. It now uses "Crystal" and announces the phone number for the MBTA police.