I think this "spam-can" stems (specifically in the UK) from the fact that the clubs / ATOs no longer own the aeroplanes they operate in many cases. The company who own the planes, do just that
Whilst I would agree with your thoughts on the general state of flying school aircraft I think you will find the term comes more from the fact that these are made of tin rather than the general wood and fabric of earlier aircraft, add in the fact that they are relatively easy to fly so what is inside the tin can can be fairly mediocre and you get the term!