Put it like this. If the Birmingham to Manchester Airport is such that everyone gains by it, why did it not occur twenty years ago?
That would apply to stop any ideas from coming to fruition.
Who knows whether this will be a good idea. The Airport will have some sense of their passenger origins and could have data supporting a service. There has also been a growing recognition of changing economic geographies.
We used to think going from Cardiff to Swansea was completely mad. Look at the road network before they bridged the River Neath. Now imagine that there was neither an A48 nor now the M4. Now I drive to Cardiff and park within 45 minutes. Cardiff is my City Centre of choice and Swansea decays.
The concept of distance continues to evolve and that means Manchester Airport is now on a bigger radar than it used to be.
The train service from Birmingham to Manchester Airport is simply part of that picture.
Note that the service is also a lot of other local journeys and is not just about Manchester Airport. It is just that in this case there is a natural terminating point in Crewe that could be used in a way that Birmingham to Stansted Airport does not have a natural terminating point amongst the constraints of Cambridge station.