If ATC could as the rule rather than the exception advise how long until push back in these delay situations, then this would give the handling companies time to send the tugs to the correct a/c, but would doing this mean much more work for ATC ?.
Also we in "company" are able to choose which a/c we want prioritising for p/back if 2 a/c are due off at roughly the same time and using same tug - we don't just pick one for the hell of it though, we look at multiple other factors first when making the decision.
But ultimately, Communication is the word - ATC advises F/deck of estimated push time, F/deck then advise Company/handling agent who can then better use the resources they have to ensure everyone is happy.
Simple really.
