I think the ROS Discourse and ROS Answers sites inherently serve different roles for the community, and do not think it would be wise to merge the two. However I do think ROS Answers could use an update:
As for migrating ROS Answers to a newer platform such as stack exchange or a separate discourse instance, I’d recommend reviewing the thread above. There are perhaps a lot of logistical challenges though, such as maintaining/archiving URLs redirects, linking user accounts, migrating tags, etc.
I’ve seen other sites use discourse as a Q/A, although I think it’s a little too linear, and doesn’t fit well when there may be more than one correct or top answer, or when answers may change over time.
I also prefer the threading model that stack exchange (and AskBot before it) use, allowing parallel discussions to nest under comments for each submitted anser; it’s just a better way of organising answers for later readers who don’t want to slog through an entire thread just to find the other assortment of potentially interleaved solutions.