That a user has not been around for a while does not mean he'll never come back. Sure, we get a lot of drive-by posts from users who never return, but I hold out hope that one of our top users will yet return someday, and I sure wouldn't want him to find his account gone when he does. Inactive users are still users. Also, when you come across a user who's given some good answers (or asked some good questions), don't you sometimes want to see what else that person has posted? You lose that if the account goes away.
(I trust you are only talking about the users and not their posts. Even when a user is deleted, his posts remain -- for good reason. This site is about the content, not just the users.)
As for the community accepting an answer in the absence of the original poster, this has been raised before and declined. The acceptance mark belongs exclusively to the person who asked the question, and he is free to not award it.
If you want to prevent questions from being bumped by Community, help them get a good answer. Community doesn't bump questions with positively-scored answers, even if no answer is accepted. The purpose of the bump is to remind us of questions still awaiting answers, and to make newer users (who didn't see them when they were posted) aware of them.
Community also doesn't bump closed questions, so if the problem is really the question rather than its (lack of) answers, you can advocate closing it.