Logic
Brain Fade :
There is certainly an issue with extra manufacturing impact of more tugs. However I think that once an airline has obtained the extra tugs needed they will eventually re-coup that impact with the savings. We are talking about a huge amount of fuel globally. Additionally once airlines get organised they should be able to do this with less increase in tugs than is suggested, as a tug's utilization rate is not that high (ie they sit for some time after each aircraft they push/pull in.) Obviously depends on airport and airline and airline flight frequency. Perhaps pooling the tugs would be a way to go.
I think initially it would work well to tow out but not so for towing in (unless waiting for a gate and then the tug should be sent out) We burn 1.5 tons parked on a taxiway with one shut down, while waiting aprox 40 mins for a gate.
As far as engine wear goes, might not need 20 mins as sometimes our taxi times are very short and we launch in under 10 mins from last engine start.
Like the cable idea !!
Cheers
FG