Motors don't really "wear out". Unless you have internal temps above 350F or so, the magnets will never loose their magnetic properties and the coatings on the windings won't melt. Replace the bearings occasionally and they will last for ever. I have scorpions that are well over 5 years old, thousands of flights, they still run like new. Some have even seen water, fire and more!
Sounds like you've just put more load on the motor as you've progressed over the years. I'd suggest dialing it back a bit with more efficient collective management, or get a bigger motor.
I love my 4525-520 in 700 electrics!
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