Over the years, I've noticed that practically all electric fans using shaded a pole motor start running slower and slower over time... until they all eventually fail and won't start anymore... most of these motors don't even have a start capacitor... And when they eventually fail, they don't burn up in flames or dramatically go up in smoke or anything, they just die off... Also, their bearings don't freeze or fail either (although the ones with bushing bearings tend to start vibrating a little bit due to normal wear).
Does anyone know why these motors stop working?

 

I have generally found it to be the (Oilite) bearings wear, they may overheat and develop a intercoil short over which affect the running efficiency.
The windings are sufficiently high an inductance that they rarely blow a fuse when seized.
Max.

 

Higher bearing friction, due to wear and drying-out of lubricant?

 

I've never had one get an attitude and just quit...it's always the bushings.
And they don't burn up because the windings have so much impedance that they won't allow enough current to make them smoke. (The label says, "Impedance Protected".)

 

The bearing lube gets stiff and gummy and the motor doesn't have enough grunt to turn. I just to had fix one fan in a bathroom yesterday. Wouldn't rotate, could turn it by hand but very stiff, sprayed with WD40 and away it went no problem. Had similar experience with these type of motors over the years.

 

The bearings are often made from sintered bronze (Oilite) and are porous and are initially soaked in oil, sometimes it pays to remove the bearings and soak them in solvent, let them dry and then soak in lubricant.
They are also usually cheap to buy.
Max.