Like many Cross Country, and indeed lots of other rail services in the UK, this route is not about end to end travel, it serves lots of different flows, some of which will be end to end but many will be between intermediate points, which may overlap or adjoin so the through train is very useful.
Yes you could say that people in Birmingham don't go to Stansted Airport, but those in Leicester or Peterborough may well do.
Its fine to say "there are other trains from Cambridge to Stansted" but when there is a family of four with luggage this is not an attractive option, even with a same platform interchange at Ely.
Fully support extending the second XC service to Peterborough, and indeed on into East Anglia.
This presents several options, including but not exclusively:
*Bi-hourly to Norwich
*Hourly to Norwich
*Hourly to Ely then bi hourly to Norwich or Ipswich, the latter in the opposite hour to the GA path and assuming that GA does not make their train hourly.
This would be a further boost to the Breckland line, but if this meant too many trains between Ely / Peterborough & Norwich then they could always use the West Curve as the loco hauled services did until the late 1980s .