This. I'm not a fare expert by any means, but my understanding is that a huge selling point of NDC/"branded" fares is that they're decoupled from the GDS fare buckets. While full flex might always book into Y/J for operational reasons, I'd expect in most cases everything else would book into whatever lowest bucket is available, and as long as the flights are booked through NDC with a branded fare the underlying fare bucket won't matter at all. Both AA and LH already do that: