PSA: "Necroposting" is allowed. If you have a better answer to an old question, something new to add, an error to correct, etc. then it does not matter how old the question is. You are allowed to post anything that will help people reading the question in the future.
What you aren't allowed to do is post random off-topic comments on questions, including both new and old ones. We're aware there are a few new users doing this currently. We've given warnings to them and hidden their posts, and we'll keep doing that. If it doesn't stop, then they will be banned.
There doesn't need to a dramatic change in response to a few problem users. Closing old questions means the questions can never be updated again, which is a problem for a site like this that seeks to give the most helpful and correct answers. It would also be an extreme moderation burden. It's not the root cause of the problem, so it doesn't need to be fixed.
