This problem is in a lot of places. Many times though it may not even be a deleted account, but rather an account that hasn't been logged in for a few years.
I think that at some point(6 months? a year?) a question should be up for nomination of an answer. For instance, using the new moderator flagging ability, add the "I think X should be the accepted answer" option. And when there are say 5 votes for answer X, then it is put on the diamond moderator queue. If they think it should be the answer, then that answer becomes the accepted answer. Also, there should be text that indicates the answer was community-selected. Of course, if later the asker comes back online, then they can change the accepted answer.
This specific flagging ability should also have a reputation requirement. I think 500 or 1000 is a good amount.
My approach is intentionally slow and requires a lot of "process". This is because we really REALLY want the asker to choose the accepted answer. But, if the user is no longer available, then they should let the community choose the answer that's the most useful to the community. And just to make sure that people aren't trying to assign an answer for a question that is better off without an accepted answer(such as closed questions), it requires diamond moderator approval.