DD is essentially for those who get waitlist in Brindavan AC.
That's how it started, that's how it's always been. SR even removed ACCC from Brindavan to incentivise DD.
Now that DD has been established, Brindavan's ACCC is back and now it's also faster.
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more...
The only good thing is that DD now has solid patronage, with most of the middle class passengers preferring it, due to its Brindavan-like timings but AC.
So I guess SR will now try to shift them (and the upper middle class Shatabdi passengers) to try and board Vande Bharat now, by running it in a similar slot.