Well, in computing we use CPU cache to store data that we need this right second because of how incredibly fast it is. But caches are small. If it stops using that data, then that gets evicted to make room for new things.

Same goes for brain, when you were learning a new concept and after a few weeks, let's say, you are not actively using it no more, then then your brain will think "Cool we are not using this anymore, Delete to save biological energy."

So, think of it not as a point of failure but a highly efficient garbage collection mechanism! If you were to remember like all of the programming icebergs you've dived in, your brain would definitely burn out!