Title says it all (or at least should)… then there is original post which should define “what the topic is about”.
When topic gets broader in time and it makes sense, I usually just reflect it by changing the title to more broader one.
I agree that splitting late can create more troubles than it solves… I always weight pros and cons before taking action.
But as I said before, when you split the topic on time (after say two or three posts when it is clear that this won’t be just short diversion) this problem won’t happen.
You are right… That is why we struggle and moderator’s job is hard 
… But it is not reason to let it go. Can you imagine how would forum look without any structure into forum categories and topics? It would probably look like chat channel or facebook. Neither of those can serve as knowledge archive. People waste a lot of time there searching for valid info. But forums - when used right - can be great knowledge bases. You can learn from past threads and the only thing that bothers you is how fast you can get the information - and that is usually about filtering heap of off-topic posts.
In ideal case, before starting new topic member should look with fulltext search if there is not similar topic, then read it first and decide if reply to existing topic or start new one. There should not be duplicate topics…
This is also why we differentiate between garden topics and theoretical topics… Garden topics can drift in time to whatever OP wants to… Theoretical topics should stay focused and open in time to add more knowledge if future brings new discoveries.
There are also areas in which this policy is not applied so strictly - for instance in Smoker's Lounge
Trying to catch offtopic early by friendly suggestion from moderator to stay on topic or splitting it early solves all the problem that you have described.
Instead of resigning, please participate and help us make the forum better - every member can flag any post as off topic and help to keep it organized…
Following rules should be a standard when using any forum:
- Don’t start a topic in the wrong category.
- Don’t cross-post the same thing in multiple topics.
- Don’t post no-content replies.
- Don’t divert a topic by changing it midstream.
If we keep habit of this small hygiene we can prevent a lot of issues that can be very painful… For those who want to just chit chat, they better check our Discord Channel…