Only the person who asked the question can accept the answer, so it would be impossible for the accepted answer to stand as the consensus unless it is also the most highly voted answer. It would effectively give the asker the power to stand as consensus. It would also imply that there is no consensus if the asker never accepts an answer.
Using the most highly upvoted answer as the consensus makes much more sense because everyone can vote, but only once. More generally, the highest upvoted answer (or question) can signify consensus or the most downvoted answer (or question) can signify consensus that the post is a bad idea and that we should do the opposite.
The asker's acceptance can be considered an upvote just like any other. This would give the asker an "extra" vote, but that can be counteracted by the fact that everyone else can vote on the question itself and get an "extra" vote that way. If the question is in the form "should we do...?" then there is consensus that it is a good idea if it has a high score and consensus that it is a bad idea if it has a very negative score.