The definition of Necroposting currently in the R&R says "The posting in a thread in which the discussion material is more than 2 weeks old and an insignificant contribution is made to the thread."
While the forum can easily determine whether 2 weeks (or 3, or whatever time limit is decided upon) have passed, it can't tell whether the contribution is significant. So, saying this rule can easily be automated, to save work for the moderators and infractions for users, is inaccurate.

The scenario that most worries me is one where someone posts a problem or question, and no-one has a good solution or answer, so the thread dies. A month later someone finds the solution but can't post it... They can make a "follow-up thread", but the forum's Search function is so flawed, a person looking for a solution for the same problem might easily find only the problem-statement but not the follow-up thread containing the solution.

As for constructive suggestions: Is it possible to make it so threads older than X don't get bumped when updated? It works that way on another forum I know, but that one uses a proprietary engine that is nothing like vBulletin. If we could have that here, "necroposting" ceases to be a problem. People can post in old threads if they want, it does not interfere with anyone else's browsing.