I agree, many questions remain relevant long after they are asked and long after the original asker is satisifed with the answer. And the value of some answers change over time (e.g. "what is the best restaurant in ..."). This is why you can always post new answers to old questions in Yedda.
The "do not close blog yet" feature you've mentioned has to do with the amount of proactive exposure this question will get, however, it seems like this is one of these features that no one really cares about, and we'll probably remove it soon...
To summarize, what you referred to as tag-matching indeed never expires.