Century old motors were well built! And probably conservatively designed because electricity was new; they didn't know which corners you could safely cut.
In those days, everything mechanical was designed for easy maintenance; nuts, bolts, taper pins; simple tools to take the whole lot apart, adjust to take up wear, reassemble and use for another 10000 miles. Run out of parts? Turn another one to fit!
I had a 1910-era lathe still capable of turning within about 0.002" (traded it for a 1928 model!) and my 1840s watch is keeping very good time.
In an era of relatively cheap labour and expensive materials, this made sense. Who knows, we may end up back there some day!
Meantime it's worth studying how things from another era are made; partly to keep the skills alive and partly because good engineering is good engineering, from any era.
Just to clarify because this seems to have hit a nerve : I'm not simply equating long life with good engineering. What makes these motors good engineering is the skill with which they met their design goals using materials and techniques available at the time.
And long life was almost certainly one of them; reliability (not measured as MTTF but the ratio between MTTF and MTTR) i.e. easy repair, and efficiency. Swapping motors for a fix is not the issue; replacing brushes, re-lining bearings or (major job!) rewinding the motor was what happened - and what the motors were designed for. It's NOW we kinda-sorta-fix things by replacing motors.
We haven't improved THAT much on 92% efficiency in a motor in the last hundred years, but we do it with a lot less copper and iron. We can equally well admire a modern brushless motor with sealed bearings and no maintenance for ten years; they can both teach us something.
Your breadboard diagram shows the 9V battery shorted. That is, its terminals are connected by nothing but a wire (internal to the breadboard). This doesn't accomplish anything except make the battery hot. There's no way for the current from the battery to go anywhere except through this short wire. Notably, there's no way it will go through the motors.
The only power source for anything on the breadboard is coming from the GND and +5V connections on the Arduino, which are not sufficient to power the motors, because they are not designed to supply much current.
You need a voltmeter to do electronics. Trying to troubleshoot a circuit without a voltmeter is like trying to repair a car with your eyes shut. It's impossible to say exactly what is wrong with your circuit if all you can know about it is "it doesn't work".
Best Answer
'D' shafts are not appropriate for plastic gears because the sharp edges cut into the gear hole and chew it out or break the gear apart, whereas a round shaft with slightly larger diameter will fit tightly and not damage it (provided a good quality plastic is used).
'D' shafts are used on 1/10th scale '540' size R/C car motors, combined with a steel or aluminium pinion gear and grubscrew. This is required due to the high torque load which is too much for a plastic gear, and greater convenience when separating them compared to a pressed on metal gear (which needs a gear puller to remove).
They are also commonly used in small appliances such as hand drills and electric screwdrivers, often coupled to a molded metal pinion with matching flat. In this case the pinion is simply slipped on and held in place by the gearbox, so is easier to assemble. The down side is that a larger 'flat' is required to avoid chewing out the gear hole, which weakens the shaft. This technique is also sometimes used in devices such as hair dryers and vaccuum cleaners which have a plastic impeller, with a metal clamping ring added around the outside to make a stronger connection and prevent the plastic from splitting.
Another downside of the 'D' shape is that the coupling may not be perfectly concentric. For applications that need good balance, such as model airplane propellers, a Collet adapter is often preferred. This clamps tightly around around the shaft while ensuring that the output shaft is precisely centered.
In reality small electric motors are almost always manufactured with round shafts, then ground down when necessary to produce the 'D' shape. Motors sold for general purpose use don't have a flat because the customer might not want it, and they can always grind or file a flat into the shaft if they need it.