The biggest enemy of trolling motors is using them with low battery voltage. The lower the voltage the greater the heat. Bushings expand, brushes glaize over, seals get soft, ect. And when one contact gets poor then the ones downstream of it begin to degrade.
Bottom line:. If you are running your battery(s) below 1/2 charge during your outings then you are gradually killing your TM. Usually one size bigger TM, fresh batteries, and some knowledge on charging is what everyone who suffers from TM issues needs.
If you never run them on low battery voltage those things will last forever and never get weak.