It is not a wish list, but just suggestion
If a thread keeps going for so long, would it not mean that the topic is hot, controversial and interesting, if not informative? Forums with these long running threads, I used to rate as, are active and growing forums with bright future. Forums with only 2-3 posts, and getting quiet all the time with no new threads or posts are dying forums, I reckon.
And forums in the ancient Greek times, were open space where all the people interested in philosophy, religion, politics or divination used to gather and discuss the topics on warm sunny days, just like us doing these days - only difference was they didn't have internet and phones, and had to make personal appearance to the discussions.
I am sure in the middle of discussion, there would be times where posts / debates veering off to some sideline issues, but it is just natural and humane - often happens when the members get close to each other because they feel they found someone they can relate to or agree absolutely or just feel friendly to each other - can happen. If it is not too drastically crazy off topic stuff, I would just leave them alone, chances are they will pick themselves up, and return to the topic, I am sure.
I can see Moderator can be a difficult task, but at the same time it is an important job which must be managed properly with strict rules, set procedures, clear agendas and ethics, otherwise forums turn to just an ideological sanctuary with some handful of cliques adulating each other, and where moderators play a Kim Jong-Un.