I think there's a huge tragedy of the commons when it comes to RAM. I get all the arguments about unused RAM being wasted RAM, and that it is more effective to drop a cache at the last moment, just in case it would be used before then. But those arguments only apply if you have a single entity handling the allocations. Otherwise the program holding the memory has no idea that it should drop a cache to allow a different program to use the memory. If the single program is the OS, great. If there's a single large program using RAM such as a browser, then it's still workable but not so great. If you have more than one program each trying to eat 80% of the RAM available, then you're in a world of hurt.

And of course, every program thinks that it's the one and only important program running on the computer, which is how we get into the situation we're in.