There are other problems associated with a tug assisted taxi.
There will need to be a large start up area off one side of the taxi way as aircraft often have problems at start-up that necessitate a return to the gate. On the apron rarely a problem but if this all takes place near the threshold then an escape route for both aircraft and tug is required and many airfields simply don't have the space, just one aborted start-up and the ensuing log-jam will take a while to sort out.
Another consideration, how many ground engineers are you going to need, at JFK for example, to accompany each aircraft to the start-up point and then make their way back to the next aircraft on their list? Are you going to provide facilities at the start up area so that engineers can remain there for a shift to overcome this? If you do you have just doubled the engineering requirements, if you don't your engineers are going to spend an inordinate amount of time dedicated to one aircraft, to the detriment of others that may require them.
The list goes on, any system that requires tug, driver, handlers and engineers to be at the start up area at the threshold is going to double your workforce and tug requirement or, if not, it will halve productivity/take offs etc. and just think of the chatter on the ground frequency that will eminate from two dozen or more tugs all roaming the area!.