I new that it was possible to reply as a new topic and spent a quite few minutes trying to find that option, far longer than I would expect most users to spend. I then spent quite a few more minutes fruitlessly searching for the term “reply as new topic”.
If I am writing a response to a post then I am replying to that post, irrespective of where my reply is found. The suggestion of explaining to users that what they are actually doing is linking to a post as opposed to replying is rendered ineffective by demonstrably being crazy-talk. If that is how you want to use the word linking then I do not think it means what you think it means, or at least, I do not think it is used how I think it is used.
By far the best way to encourage XXXXX is to make it easy and obvious! I thought that was an underlying philosophy of Discourse.
If a user wants to reply to a post and then decides to replay as a new topic then they are in effect helping to curate the forum at the expense of their own time. There is no immediate benefit to them, they are making the altruistic decision not to derail an existing conversation.
Once they have formed that preference they have a finite amount of motivation to do the right thing, they are not going to faff around copying and pasting links as you suggest, they are either going to click an intuitively named button in an obvious place that is available “just in time” or they are going to give up and never try again.
If that option is going to appear “just in time” then it has to be part of the reply dialog, not something that has to be clicked before replying. Otherwise you are asking the user to decide how they want to reply before they know if they want to.
I really can’t see an issue with giving “replay as new topic” near parity with “reply”. Even if it only gets used a very small percentage of the time it should be seen to be a good and valid option for users. Perhaps more importantly, if it is hidden away, no one will know it exists! The aspiration for a largely self-moderating space for civilised discourse doesn’t work if users still use it like any other forum software. I have a chrome tab open next to this one for a forum running phpBB and every single topic seems to get derailed. The users don’t know any other way to use a forum and the software doesn’t give them a viable alternative.
Discourse has some perfectly good tools for dealing with troublemakers but so does pretty much every other forum software. The really intractable problem is getting users to talk (and listen) without shouting over each other and Reply as new topic is pretty much the only de-derailment tool that Discourse seems to offer users. It seems utterly bizarre to me not to want it front and centre of the user experience.