U self answered query...when already ICE/Shivnath cater the distance, y there should be an illogical extension, which gonna keep Jan-Shatabdi unclean & late!!
With 7 hrs journey, its total 14 hrs run...add 6 hrs primary maintenance break & it adds to 20 hrs run, with 4 hrs TAT @ secondary depot & slack to control late run...
2.5 hrs journey for NGP-G will add some 5 hrs run in train schedule....already train gets 20 hrs booked & u add 5 n make it 25 hrs,,,first, from where this extra 1 hr be adjusted...
more... in total 24 hrs of a day...further, how train gonna manage some TAT @ secondary depot (Loco Reversal, late run TAT etc)...
With all this, extension is just impossible & other two trains are perfectly catering demand!!!