This would make sense if a user manually searched for a topic, found it, and intended to resume discussion. On the other hand, being tricked into thinking you’re replying to current discussion when you’re in fact not is bad design. There are many reasons why you wouldn’t want to resume an old topic. To name a few:
It’s no longer accurate
For instance, let’s say I find this topic in Suggested Topics. I might go there to post on the issue I think is only a few days old with “Oh, I was having issues with password resets not working too about a month back” to hopefully give more information about the scope of the issue. Oops! It turns out this issue was not recent and was already fixed – now I’ve pointlessly bumped the topic up to Latest for everyone to see.
Another common case is me finding a help request and posting a solution that was made possible after the topic had been posted. Yes it’s good to have that information come up for whenever someone has the same issue in the future and finds this topic, but is that worth bumping the topic up to Latest, especially when similar, more-recent topics would also be found in the search that contained the new solution?
It’s no longer relevant
Imagine I’m on one of the food Discourse forums. I navigate to a topic showcasing food that members are making for this week’s dinner and post “Look what I made! < insert image >” Oops! The topic was from a couple months ago and the virtual bake-off this topic was meant to host is no longer relevant. The same thing can easily happen with any “Lounge”-y topic.
The forum has moved on
Let’s say there’s a friendly and constructive back-and-forth disagreement on one particular topic. PersonA has one opinion and PersonB has another opinion. I reply thinking this is recent, but in fact both parties have since moved on. It’s pointless to bring out the skeletons in the closet as nobody agrees with what they posted earlier. It may be worthwhile to resume the discussion, but I should be fully aware I’m resuming a long-dead discussion instead of resurrecting it assuming it’s current.
The members on our Discourse forum have had nothing positive to say about Suggested Topics, and I don’t think it’s working how you envisioned it to.