The argument is true for a sophisticated airliner with and IRS that can sense things like the earths rotation, coriolis effect and so on and so can tell well true north is. A remote indicating compass can be used as an alternative, and it will sense very accurately, the direction of magnetic north and is a cheaper alternative.
For a great deal many other aircraft such equipment is not fitted. Where would you put an IRS or remote sensing compass in a Cessna 172 for example, or other light aircraft, and how much would it cost?
Although the magnetic poles move, the rate of change is so small as to have little effect. We are talking the odd degree a year. Now most people cant fly to an accuracy of 1 degree over a period of sixty seconds, so you can see the effect is neglible.