You got all that switch when going from S5 to S6 because of a full reowrk of the XP system, so it was logical you could not retain the legacy boards from S5 to work in S6 or eternal.
The XP rework was a game wide change, not only for the expansion.
You could carry your old legacy gear, but to have legacy XP boards was a bit unrealistic…
All the D franchise has gone that way, when a major rebuild is done, so nothing new here…