a) I wonder in which sense HDFS was ever the most popular DFS. You could argue that it is not even a DFS, because it is not a general purpose / POSIX file system. In 2012 there was Lustre, GlusterFS, GPFS, ... Not many DFS were built since 2012.

b) It is completely unclear how the replication works, and which properties it has (split-brain safe?). From what it states (see citation), the "repair" is not automatic.