Well, as others have said: Hitting exactly 80% is pretty much impossible. And we've established that hitting more than 80% produces languages that are often bad, in some way or another.

So, logically it follows: aim for less than 80%. You can always add things; you can't take things away.

> And we've established that hitting more than 80% produces languages that are often bad

No we haven't; we've established (for very dubious values of "established") that aiming for more than 80% is frequently not worth the trouble.

Sometimes by aiming for less than 40% you add the _wrong_ thing, which you need to take away before you can add the right thing.