I guess you completely missed this : BG + SG are worst possible combination for "high speed railways (200+ kmph)".
I can't see any "impossible" word being written here, and the case is specifically for "high speed railways". For goods and industrial sidings, any combination of gauges is possible.
Anyways I'm done with this discussion. You may file a PIL or Grievance to Ashwini Vaishnav and Ministry of Railways for your valuable suggestion. They'll be glad and enlightened.