Cornwall has an HST▸ in each direction every two hours or so, and they are often virtually empty. A 153 is perfectly acceptable for a 2 hour journey. Maybe not enough capacity on its own but Cornwall local jobs don't need fast trains as they can only go 75 anyway,
As long aas the 153 isn't on it's own!
It used to be a 2 car 158 and was replaced as far as Plymouth by an HST which used to start at Plymouth.
During summer the Portsmouth service was often 3 car and still busy.
Was it? I never noted it as that!