This is incorrect - electric motors draw more current when under load. I would suggest researching this or playing with some toy electric motors run from a battery through a current meter.
Which is a true statement but incomplete and therefore misleading. A fan motor is under the highest load when it is doing the most work. That is, moving the MOST air. If you restrict airflow (e.g. with a dirty filter), the motor is under less load and draws LESS current. A lot of people get this wrong because it's counter-intuitive on the surface of it.
You can test this very easily with a box fan and a kill-a-watt. Turn the fan on High in the center of the room and read the power. Now move it against the wall. You will hear the fan get louder because it is spinning faster, because it's doing LESS work. The meter will also show less power being drawn.
It is true that a dirty filter restricts airflow through the whole HVAC system. This extends the system's "on" cycle, which reduces the whole system's efficiency. But the popular claim that a dirty filter will burn out the motor is bunk.
Sure, some motors have characteristics that can be tweaked to run at different work loads either with PWM, or allowing them to pull more amps to drive higher loads. Hell, you can even overvolt them and make them actually work harder.
That's not how HVACs work though. Have you ever opened one up and did a repair? The circuitry is dead simple, there's no current limiting setup or PWM to control how much the motor is pulling or spinning, there's no CFM measuring device to give the motor more volts or a higher amp limit.
The motor is simply pushing air. Air isn't something like a solid load where a motor might lift something or move a lever or gear, it's fluid. The motor is going to run at max and be done.
Back pressure doesn't impact the load of a fan motor, at least not as the scale we see in HVACs, we're not making vacuum chambers.
Anyway at the end of the day I agree with the original characterization of wear and tear - motor and blower will probably outlast most other components. In the case of the one I mentioned this seemed to be true. You just may end up paying a little more for the electricity to run it.
Apologies for the tone!
> Have you ever opened one up and did a repair?
Well funny you should ask, but yes! Two times. One time, a power relay on the control module shorted and blew a hole in the circuit board. Had to replace the control module there. Another time, the start capacitor needed to be replaced on the giant 2KW squirrel cage motor. I removed the blower, disassembled it, lubricated moving parts, and of course tested it out on a stand. That much blowing power is quite impressive when right next to your face.