Topic shifts happen, as much as the admins wish it didn't. This is the inevitable result of people being expected to support their assertions.
The topic approval system makes it even harder to shift an off-topic but lively discussion into another new thread; even before that, most debators would use the shift as an excuse to avoid continuing the discussion. Very rarely did all participants follow over to the new thread.
Very often it's happened, too, that the best discussions have been off-topic. With the Free For All forum gone, there's no longer a place for topics to range as they will, to the detriment of all of us.
So, let's get over ourselves, admins, and let threads shift topics. I propose that threads be allowed a maximum of three topics; if an off-topic aside develops into the main topic, let the thread title be changed to reflect that new topic. If topic descriptions are kept brief, then topics can easily have titles that reflect the three topics found within.
When the thread hits the fourth topic, close it down. That way we never hit the 300 message limit, or do so very rarely.
Threads wander. It's a necessity of having to support our statements, and of having to rebut our opponents supporting arguments. So let it happen, within reason. My suggestion, I believe, does just that.