It's about time Metro got some high-capacity buses of their own.
The problems with that sort of arrangement, despite looking good on the surface, is that they need to be used rather than sit parked up, they need to be near to any given incident and they need drivers available and not exceeding their permitted hours to drive them from any A to any B on demand.
The costs and logistics of that are horrendous.
Would you have drivers idly passing the time of most days in case something happened? Where would you have these buses kept? They are of little use in Ringwood for a blockage at Newport for example simply because of the time taken to get where they are needed. And how would they earn their keep when not in use for rail replacement? Machines need to work; they can be less reliable (including having low batteries) if they are not in regular use. True they could be employed on weekend rail jobs when track work takes place but again there is probably insufficient work to justify such investment when taken across a year.
Then there is the minor point of Metro not being an accredited bus operator ............
It is mightily inconvenient when a like is suddenly blocked especially in peak hours when buses are in very short supply due to their need on school and route duties. It is also inconvenient to lose a family member in a collision with a train. There are - in the main - no bus routes closely paralleling the rail lines meaning there is often no scheduled alternative transport but the powers that be do what they can as best they can within the limited scope available.