What happens to users who have not seen their accounts for more than a couple of years? Do their accounts remain intact? I wanted to know so because, there are often a few questions with about 7-8 votes but no answers posted, by users who haven't seen their account for a couple of years. Will it be of any help if I answer them? More importantly, often before giving a good answer, one needs some simple clarifications about the question itself; but in this case, what am I to do if the O.P is not available?
If a user has at least one visible (non-deleted) post on the site, their account is preserved indefinitely. However, this does not mean they will ever use it again. So you should not expect any notes of gratitude from that particular user if you answer.
And you can expect is that an unanswered question with 7-8 votes is of interest to other people. Those who upvoted it, and many more who visited but did not have the privilege to upvote (you can gauge their number by looking at Views count). Additionally, when a question gets a good answer, this boosts the visits long term, since it gets inbound links over time. You may end up helping thousands of people, even if the author of the question never sees your answer. You will never know who they were... So, to have a visible token of appreciation, the site gives Revival badge for the first answer to an old unanswered question (provided the answer gets the score of 2).
As for clarifications: if the question cannot be answered because it's unclear, vote to close: "Unclear what you are asking". If the question never gets fixed, it should eventually be deleted: there is no point in having unanswerable questions lying around.