If a user asks a questions and gets a proper answer and the user becomes inactive forever or deletes his account without accepting the answer, then does any other person has a privilege to accept the answer other than OP? If no, then how to deal with such questions?
No one1 can accept an answer except the OP of the question.
In general, it doesn't matter since an acceptance only means that the answer helps OP the best.
Accepting an answer is not meant to be a definitive and final statement indicating that the question has now been answered perfectly. It simply means that the author received an answer that worked for them personally.
However, Stack Exchange is a Q&A site for public. It strives to help many people, not only the OP. This is indicated by the voting, which is more important in long-term.
The voting is also used by the system to indicate if a question is "unanswered" or not: accepted/upvoted answers.
1. Except Stack Exchange employees directly accessing and modifying the database, but that's never recommended.