And the nose lifters are rather more expensive than a "classic" push-back tractor. Heard somewhere that a Douglas nose lifter capable of handling anything up to 747s are around 2.5 Million Euro's a piece. Oh, and a 747 capable Douglas is too big to fit under the fuselage of MD80/737 types, so it's not really a case of "one size fits all".
The "classics" will be with us for a long time yet.
PS
A nose lifter still needs a guy on the headset during push and start. However, subject to company policy a nose lifter may position aircraft from A to B without the presence of a guy in the cockpit. Or at least, that's what the glossy brochure says. I'm left wondering how the parking brake will be set/released without someone in the cockpit, but that's a different story.