<<Pushback & start should not be given until there is a clear run to takeoff.
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Not possible at busy airfields; there are too many variables. You need a fair few at the holding points to enable expeditious departures by taking advantage of departure separation standards. Too many heading in one direction may cause delays.
Not really relevant to this thread but the time from pushback from each stand to a taxiway entrance for each aircraft type will be a constant, as will the time spent on each taxiway.
It would take very little computing power to generate the timings to have each flight arrive at the threshold holding point on time for its slot with a nice orderly line correctly spaced out to follow.
I agree that it is nice to have the next one holding ready but it is inexcusable to have a logjam of aircraft sitting still on the ground with engines running. It is even worse to have aircraft leaving a stand when it will only be joining a logjam.
Some airports are just plain disorganised.