I totally agree. The ones who can only understand hakka and taiwanese are most likely in an old folks home or gone. And wouldn't be travelling the mrt by themselves.
I say Mandarin only is best. With English written on the boards.
BART only announces in English (and has no electric signs). Works fine.
If Taipei feels it needs English announced as well (totally not needed in my view), then so be it. But that should be it.
Very few english speakers are blind in Taiwan and can not see and read the electric signs !