You are telling me Shatabdi was not possible between Delhi & Varanasi?
Although I consider the acceleration factor, but the MPS remain same, so the difference is not that huge, not to forget the less slack(buffer) provided to it. e.g. Rajdhanis between Delhi & DDU (similar distance) take equivalent time like T-18 between NDLS-BSB
Moreover, Almost all news releases portrayed T-18 as a replacement of Shatabdi, and given it's concept, it even makes sense.