Can a passenger jet be towed to safety if it breaks down in midair?
Towing is quite feasible, and even landing while in tow is possible - I've done it myself in gliders. The hard part is catching up with the stricken plane.

To stay with Keegan's Boeing 777: It most likely will travel at Mach 0.8 and somewhere between 30.000 and 39.000 ft when disaster strikes. Let's further assume that power fails instantly, so the aircraft, traveling at 240 m/s, will develop a sink rate of 15 m/s just to continue flying. When it sinks, the increasing density allows it to slow down, so at sea level it will fly at 130 m/s, sinking with 8.125 m/s. For simplicity, assume a linearly decreasing sink rate, and the remaining time aloft will be less than 1000 sec or around a quarter of an hour.

Now the towplane has 15 min to scramble the crew, run up the engines, get take-off clearance and to actually fly an intercept to the stricken airliner. How realistic is that? How many airports are needed to have 777-sized towplanes ready, and how many otherwise unproductive 777s would this scheme need?

A car can stop and wait a couple of hours for the tow to reach it. An aircraft can not.