It's my pleasure that you have reminded me about the "Golden Days" of SDAH DURONTO when it was hauled by WAP5, I forgot that.
I also accept your view. It's simply the "NEGLIGENT LOOK" of IR(i.e ER) for not providing P5/P7 for SDAH DURONTO which was the "Regular Link" of this train once upon a time and hauled this train at a"Good Speed" highhandedly far better than it's present loco (i.e WAP4).
The following factors may be responsible for the "NEGLIGENT LOOK" of IR:
Due...
more... to the dominance of Rajdhani in SDAH/HWH NDLS Route IR has given the topmost priority to it(WAP7 IS ALLOTTED FOR BOTH RAJDHANIS,WI FI is given to Kolkata Rajdhani too) and both HWH & SDAH Duronto have become the "LOW PRIORITY" trains from the viewpoint of IR(LOCO LINK IS P4) and IR thinks that the need of "Non Stop Trains" like Duronto is redundant in SDAH/HWH NDLS Route and so they are reluctant to give any further attention to these two NON STOP Durontos and in future they may close these two Durontos and instead will increase the frequency of "Premium AC Special" which will cater to the demand of this route along with the Rajdhanis plying regularly in this route.