> Unused RAM is wasted RAM

Your actual OS is well aware of that fact, and will use spare RAM to make everything faster rather than just the one narcissistic program hogging extra. It'll cache files on disk so that commonly-used things are even faster than SSD. It'll erase some amount of old memory pre-emptively so that when a program demands a block of fresh RAM, the OS can immediately hand some over. Probably other background optimizations too.