This has been covered piecemeal elsewhere:-
  More people and cars
  Less green grass and fresh air
  Urbanisation
  Pressure on local services
  More traffic and noise early and late
  Etc. Etc.

But there would be an impact above and beyond the component parts. Chalfont Common has a quality of life in its semi-rural setting that would become faster, harder, less enjoyable. It's a bit difficult to put one's finger on, but today's Chalfont Common is more pleasant and more attractive than the NSE's planned Chalfont Common. This is true for the NSE's care residents and staff as much as it is for the local community.

Oh, and of course the transition between now and then is unlikely to add anything to the village character either. Except, perhaps, a blitz mentality during the building works of the £125Million developments.