I understand what you're asking for, and agree it would be ideal, but I really wonder whether it would help at all even if it were possible because of the "dual nature" of Groups.io that I know we've talked about here many times.
In any group where you have the majority of participants interacting via e-mail it's the e-mail subject itself that determines whether a given submission will be attached to an existing topic/thread, be spun off as a separate thread, and the like. Even if you were able to "hack out a tangent from the heart of an ongoing topic" and establish a separate topic, what are the odds that any of the many e-mail participants are going to know about that in in any timely manner. I can see, on groups with large memberships and a large percentage of e-mail participants, all attempts to do what it sounds like you're trying to do coming to naught when someone replies to the original topic, from which you've now established a parallel topic with a new title that befits it, and you are back to having that parallel topic as a tangent either at the end of or, if enough "original topic topic" responses come in after it, back in the heart of the original thread.
I am driven crazy, almost literally, by the endless amount of unnecessary (and sometimes unintentional, but often not) topic/thread splitting that goes on in many Groups.io groups which is sort of the "flip side" of your initial issue - but a related one. You'll have an original topic, say, "Amazon Echo," asking for people's opinions and experiences with said product. One person responding decides to change the subject to, "My impressions of the Amazon Echo," and the splitting is afoot. I can't make certain e-mail participants understand what havoc this wreaks for those trying to read on the web interface and what this does to archive coherence as well. I've seen what are actually single conversation topics broken into sometimes as many as three or four separate topics. I have wracked my brain for a way to beg people to not do this and, if they have a stand-alone topic that is spurred by an existing one that's not a brief diversion, to actually create a reply with a Subject that will create a topic that others can gravitate to in order to discuss it. I have decided that this is likely an impossible dream on my part.
I have made mistakes, but have never made the mistake of claiming I never made one.
~ James G. Bennet