Most memes lack the ability to self-modify while preserving the thing they're optimizing for.

Memes don't self-modify but they do definitely evolve over time and the more successful results survive and the less successful ones are forgotten (die). Most of the time they also preserve what they optimize for, see; cute cat pictures.

What is the thing Bitcoin is optimized for again?

Mining coins (and hype).

And drop from popularity soon after.