1. By comparing RNC Shatabdi to KLK/CDG Shatabdis, you are comparing apples and oranges. Patronages of trains pertaining to different regions is beside the point here.
2. Ranchi Shatabdi runs at 130 between HWH-DHN, it has the most aggressive schedule between HWH-DHN except probably 12301. And ASN-DHN is a slacked TT, HWH-ASN is even more aggressive. You mean to say that a 110 sanctioned SF will be 20-25 minutes slower than Rajdhani prioritized train in a highly saturated line like HWH-ASN. That is preposterous! Don't you think ER is going to riddle RB with stops between HWH & ASN ?
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The highest patronage of 12019/12020 are given by DHN/ASN/BKSC respectively. Now assuming that all shatabdi passengers of ASN suddenly shift to the beloved Rupashi Bangla, then there will be a dent in the patronage. But stuff like that doesn't happen in real life. You see Shatabdi and normal superfasts cater to completely different income segments. The executives of ECl/BCCL/DSP/BSP who form the bulk of the passengers of 12019/20 will not just switch over to RB. There is no incentive.
4. In that morning slot between HWH & ASN, BD is the boss of superfasts, BD caters to passengers from Kolkata suburbs who actually use it to visit their offices at DGR/RNG/ASN etc. If ER places RB in slot that is somehow overlapping BD's slot from BWN onwards then, the best chance that I can see for Rupashi is to play second fiddle to Black Diamond between HWH-ASN. Forget challenging the RNC shatabdi that won't happen, ER will riddle RB with stoppages and it will trail BD upto ASN. If ER however places RB before BD then I think RB will have a very good patronage but BKSC Shatabdi still retains it's normal patronage. It will dent BD's patronage, and given how overloaded BD is, it will be a good thing.