as acts like the relative pronoun which , but unlike which it can be placed at the beginning of a sentence. Here's the equivalent with which : A decision to do it would be extremely risky, which should by now be obvious. So you can say that the subject ("as" = "which") is implied, or even that the subject is explicit if you accept that "as" can be the subject of a relative clause.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.