I suppose that works with RAM and CPU waste, but I notice the people building gigabyte consuming irc clients aren't paying for all their users RAM, so the cost tradeoff is rather different.

Look up Hamming's course on "Learning to learn". An optimal design is not robust. You add a safety factor and this addition is wasteful in some sense (thus not optimal) and necessary in another. Do you design a building so that the toilets are optimally used? Likewise for planes.

But just because you have enough of a cheap resource (RAM) it is not a given that using it in excess is either optimal or robust.

Also, most users aren't really aware what's using up the resources - only that their computer gets slower with time. I actually wish Windows would start naming & shaming applications - e.g. by showing a balloon when the system slows down, that would say that application X and Y are consuming most of the resources.

As it is now, CPU, RAM and storage usage are perfect externalities for developers. The result of their abuse is user frustration, and reduction of the number of things people can do on their computers. That, and wasting user time. Unfortunately, there's no way that would make the developers suffer costs for wasting user's resources, so the only thing that lets us have lean software is engineering competence and human decency (both of which seem to be in short supply).