Wouldn’t you want to respond based on something you just read, e.g. someone replied, as you did just now, which created a new, unread reply and thus bumped this very topic into my unread list?
In my experience, most discussion is based on a person reading new repl(ies), then deciding to reply to that topic more or less immediately after they read the new replies.
Is that not true for you? 
I guess I’m just not following this model, sorry. Is the idea that
and
- the discussion topic has no replies at all
and I’d suddenly decide “gee, I better reply to that topic nobody else has replied to for three days?” If that was the case, assuming I previously replied to the topic, I’d go to my reply history and find my earlier reply?
It does get a bit trickier if I have never replied to the topic at all, only read it, because then I don’t have any reply history for that topic. That case, I guess, is what you’re describing?
If it was a topic I created, it already has the highest notification level (watching) out of the box, but for the sake of argument, let’s say nobody replies to it for a long time. If that was the case, I’d go to my topic history and find it there?
One suggestion: if you want to reply to a topic you are currently reading, but don’t have time (maybe you’re about to go on a long vacation, have errands to run, etc), you can set a bookmark timer to remind you a few days later by pressing the bookmark button on the post you want to reply to?
Apologies – I want to understand, but I must admit this model is a bit alien to me.