Whither btrfsck?

You're right, I oversimplified. What tends to happen is that people wait half a year after the release, then install the new OS into a test partition and test for another six months before putting the system into production.
The times are obviously different depending on how paranoid the admin is and on the type of workload.

However, there are still good reasons for waiting: The bugs you encounter while running the new software on your test partition are often different from the bugs that other people encounter while running the same software in production. If you care a lot about stability, you probably want to have both kinds of problems solved before you get to the point of no return.