The problem with the simplified phrase is that your computer is expected to run more than one program at a time.
If you are only running one program, it should certainly use all the RAM of your system.
However, your desktop, laptop, phone, tablet, game console, etc. all run hundreds or thousands of programs at the same time. Each individual application should optimize RAM usage so the whole system can work together.
Another commenter in the chain talks about disk caching, which is what the phrase “unused ram is wasted ram” came from
It’s been coopted by application programmers who don’t want to optimize their software
The problem with the simplified phrase is that your computer is expected to run more than one program at a time.
If you are only running one program, it should certainly use all the RAM of your system.
However, your desktop, laptop, phone, tablet, game console, etc. all run hundreds or thousands of programs at the same time. Each individual application should optimize RAM usage so the whole system can work together.
Another commenter in the chain talks about disk caching, which is what the phrase “unused ram is wasted ram” came from
It’s been coopted by application programmers who don’t want to optimize their software