Let me explain this again but with a different spin.

When threads, or at least most of them, get old enough you, or at least I, just don't want to see them. Items in For Sale are likely not still available after a few days. Most conversations stop being active after a week or two, sometimes after a few days, and any post after that becomes a monologue, not a dialogue or polylogue. A user can see at a glance which threads are active because the titles of active threads show up in Bold.

In the previous system threads marked themselves as inactive, which is exactly what at least this user wants. With the new system everything unread still shows up in Bold so all the threads I really didn't care about before are screaming READ ME TOO! YOU FORGOT TO READ ME! Maybe I didn't read them the last time I logged in because I wasn't interested or didn't have time. If I didn't have time but do now, I can still open them, seeing at a glance they they are old. If I wasn't interested then I'm still not interested. But now I can make them go away and stop screaming only by marking them, which requires active moves.

This is the modern world, I guess. Everything screams for attention until you tell it to go away. And if another posting happens after you tell it to stop it will start screaming all over again until told to stop again.