The rules of almost any other consumer market seem to get suspended when it comes to flying!
I guess schools see it like this - how many prospective students would have to walk away before the cost of a newer a/c would be worthwhile investing in? Quite a few I think
Or looking at it the other way, how much extra per hour would you be willing to pay, to learn in something that's newer or even worse, has gadgets in it that you can't use in your PPL syllabus.
Having said that, the biggest mistake I think is when it comes to trial/sight seeing flights where a persons expectations are much higher in terms of quality.
It's really depressing that I can't see how PPL schools or rental firms would ever feel motivated to invest in newer fleets. This is because the post PPL market seems very split between those that fly for fun but are price sensitive, so will go for cost over age of aircraft and then those that can afford to fly more often who then get a share anyway. So the poor flying club doesn't have a market big enough either way to warrant a better fleet.