I forget where exactly, but I saw an excerpt from an interview with Filoni the other day where he said that if those aren’t the last inquisitors we’ll see, they’re the next to last. He talked a bit about how as they get closer in the timeline to ANH, they’re trying to play more and more by the OT’s rules, and that everyone thinks there’s no place in that era for the inquisitor characters.
So that explains why each and every one of these characters has been dimensionless cypher cannon fodder?