Suppose you wrote a question asking to find where your mistake is. It might be possible that there are 2 answers - one showing your answer and other showing another very beautiful method and OP doesn't want to accept any. It is more likely that the beautiful method will get upvote and by your feature that answer will be accepted however the other answer should had been accepted as it is answering the main question.
Now the problem with your idea is that the answer not actually answering the question will also be upvoted.
Now see there might be two cases why OP isn't accepting any answer. First of all new users aren't aware of how to accept answers so whenever you see a good answer that answers the real question you can ping the OP to say
If this answer has helped you you can accept it by clicking the grey button to make it green showing that you have accepted this answer.
In most cases OP isn't aware that he/she has to accept answer too and so don't do. However if there is a user who is constantly not accepting answer even after several times telling him to accept politely. If this behaviour continues for 5-8 question you can flag any one question for moderator intervention but try to do that minimum as moderator shouldn't be disturbed for accepting answer unless it is very urgent. What you can do is invite the OP to chat and ask him why isn't he accepting the question and if suppose he says he doesn't want to or reply you in a hurtful way you can definitely flag that for abusive.
However do remember it is fully up to OP whether OP wants to accept the answer or not and he/she should not be forced to do so. "Accepting an answer is purely voluntary and there's no obligation to do so."
This might help you.