I agree that "accepted answer" is a misfeature; but the default is for questions to remain open to new answers indefinitely.

On the one hand, good new answers can remain buried under mediocre and outdated answers more or less indefinitely, because they received hundreds of questionable upvotes in the past. (Despite popular perception, the average culture of Stack Overflow is very heavily weighted towards upvoting. I have statistical analysis of this on the meta site somewhere.)

On the other hand, there are tons of popular reference questions that had to be "protected" so that people couldn't just sign up and contribute the 100th answer (yes, the count really does range into triple digits in several cases, especially if you have access to deleted answers) to a question where there are maybe five actually reasonable-considered-distinct answers possible. And this protection is a quite low barrier - 10 reputation, IIRC. (I'm not sure what motivates people to write these new answers. It might have something to do with being able to point to a Stack Overflow profile which boasts that you've "reached" millions of people, due to its laughably naive algorithm for that.)