For instance, I'm hesitant to respond to an old thread because I'm afraid that bringing it to the front for the entire forum could revive some low-quality discussion...
It wouldn't be reviving them at all: since what you'll be adding by bumping it is a quality addition, then why should you should care about old spam posts on the thread. When you've bumped the thread with your new reply, the discussion can then take off from there again.
It would even help to get those low quality posts deleted, when the thread is bumped to the first page, the replies that were off-topic would be visible to most users, since it probably went unnoticed when it was posted.
So this can serve three good purposes, gets old spam posts deleted, generates new discussions on an old topic, and also replies/posts that were so good and were not merited, stand another chance of being so.
Generally an overall approach shouldn't be applied to all the threads. This should be decided on a case-by-case basis.
Most of the times, it's better to open a new topic. Sometimes you should reply to the previous topic (Only if you think that cannot be discussed in a better place and you have something worthy to post)
The decision on whether to post in the old thread or open a new topic should be made base on the discussion.
As steamtyme said, sometimes it can be a good idea to pm a user when the thread have been inactive for a while and you want to reply to a certain user for asking a question or answering to a question and you think there aren't any other user interested in your post.
I don't think it's a good idea to make a post in a thread and make it visible for getting old spams deleted. It's not worth moderators time.
Also, I think that the forum want users to avoid inactive thread as much as possible. When you try to reply to a topic that has been inactive for more than 120 days, you get the following message.
"Warning: this topic has not been posted in for at least 120 days.
Unless you're sure you want to reply, please consider starting a new topic.