[lupin3ken]

There are a bunch of evolutionary reasons that your brain forgets things that aren't used. Check this article for some related to sleep: https://www.supermemo.com/en/archives1990-2015/articles/slee...

But based on the current models of memory, you seem to be correct, your brain has a couple of criteria that it uses to determine whether or not it should retain memories. Frequency (spacing) is one, novelty could be another.

Even memories learned through mnemonics can ultimately be forgotten if not reviewed.

And I think the brain keeps itself plastic by optimizing "unnecessary" information out. Unfortunately, without help we can't choose which information is removed.