I've never said it shouldn't be discussed, as I have proposed in the past a monthly posting about "netiquette" with regard to forums and mailing lists, and it has fallen on deaf ears. I think such a posting would be a good thing for reminding regulars and for teaching newbies.
I can tell you that a lecturing tone, and your post is not unique in that regard, tend to put people off immediately. Thread drift is inevitable to some extent and when there isn't a real continuity break of significance (there are related tangents) it is best tolerated. I try my best to set an example whenever possible by answering valid tangents that deserve an answer by creating a new thread with a new title when answering them.
And, again, it's of far more value for the archive if when egregious examples occur the moderator(s) take it upon themselves to rectify the issue for the sake of the historical record and to create an example of "how it should be done" when someone has flagrantly ignored that. Since I am not a moderator here I do not know exactly what "the powers" are. On one other forum (and it applies equally if your access is via e-mail) I use as much as this one the moderators regularly move posts that have landed in the wrong group to the group where it belongs. One of the nice things there is that the forum software leaves a "move marker" where the information was originally posted that shoots anyone who clicks on the topic directly to its new and appropriate location. Of course, that latter feature is not something that works when dealing with e-mail going out, but the new messages that would be generated by thread splitting does.
I worry a lot. . . I worry that no matter how cynical you become it's never enough to keep up.
~ Trudy, in Jane Wagner's "Search for Signs of Intelligent Life in the Universe"