That’s because the most recent reply defines the bump date. Reset bump date was helpful when, for example, you deleted a spam post, because then it allowed you to reset the date to the one of the last post after the spam post was deleted. But now the date is reset automatically on post deletion. It was also helpful for minor edits that bumped the topic, where you could reset the date to the date when the post was created, but edits no longer bump topics.
Reset bump date still works, for example, when the first post is a wiki and a wiki edit bumps the topic. Then you can reset the date to the one of the last reply.
@ice.d what problem do you want to solve with replies not bumping the topic? Why do you want users to be able to reply without others noticing? Maybe a topic list ordered by date of post creation would solve your problem.
It’s used in the Announcements > Blog category here on Meta.
This is what you see when you visit that category:
But after ordering by activity you can see that there was a recent post in “Free Hosting for Open Source”:
