Would be a hell load of trouble just to get a user their (deserved, correct) 15 rep.
Ask Ubuntu works over time already, answer a question in a good manner and you will keep having reputation incoming for a very long time.
A question that as an answer with at least 2 up votes is already considered answered, so statistically for the site it self this is kinda useless.
In the case of a tie the longer answer gets it (they put in the most effort).
So are you saying that if a question has 2 answers:
- "hi I solved this by using something else"
- "ih I devlos siht yb gnisu gnihtemos esle."
If none of the answers has up votes and no one catches the 2nd one - and deletes it - for some time, the 2nd answer should be marked as the solution because it has 1 char more than the first? I do not see the usefulness in this, sorry. Too much automation is bad!
Giving a better answer or taking care of your old answers and improve them will score you much more reputation than getting a green check next to it and ignoring it forever.
You cannot force a user to accept an answer even if he says that it was the solution for their problem, it is also wrong to accept it for him. Each user has an accepted answers percentage displayed with their name, that should give you an idea of what will happen even if the user agrees with your answer.
Of course new users will always be a (?) but that is just a risk you will have to take when answering questions.