I'm not convinced there would be a massive saving in personnel. You would still need someone to put the chocks in, plug in ground power, A/C etc. - if you dont use the tug crew then you need someone else. Also its difficult to count all the times that the tug crew avoid a collision or delay by moving some steps or a baggage trolley out of the way which if there were no crew the automated tug would simply have to stop (possibly blocking the taxiway) and wait for someone to be dispatched to remove whatever was causing the delay.