No matter what occurred, it didn't require benching Berwyn. They already had his companion content made so that's not an excuse.
It was probably a case of having a small pool of writers who've proven themselves reliable enough to be trusted with companion content and each of those wanting to do their own waifu rather than adopting one. ...Of course, from what I've seen of the newer companions, that doesn't work out all that well, either. Characters who really don't have enough appeal (or, on the flip side, clear appeal but way too much baggage) have been made into companions and gained no traction from contributors because only their creator wants to write for them.
Part of the reason Cait gets so much content is simply because you can fit her into most any type of scene. Who's going to write more Kiyoko material with the Observer breathing down their neck or Azzy scenes when you must take into account the divisive 'she's dating someone else right in front of you' aspect?
IMO a good companion candidate should have an immediately obvious appeal/niche that has broad applications (so not something so specific that content for them -has- to cater to a single fetish; save that for gimmick NPCs), a unique but not too complex character concept to hang quest writing and scenes on (in other words: the character needs to be easily defined enough that other writers can create a firm vision of what that character's deal is in their mind without consulting a giant character lore bible), and connected enough to the story at large for them to be involved in big events. Cait, Brint, Etheryn, and Arona fit all those criteria; it's no surprise that they get the most scenes by far.
Atugia had potential, but the swing and a miss there is the lack of obvious appeal and failure to use her unique character concept for scenes. In her material it never really matters that she can take her head off. Eryka, the paladin, also overlaps with and overshadows her by leaning harder on the 'switches from rough to tender' concept that Atugia only flirts with. If these two characters had been combined they'd add up to one decent companion fit for any muscle mommy scene.
Viviane misses all three marks: no obvious appeal in a game where literally everyone has huge boobs, too disconnected from everything to justify elevating her, and too generic for writers to have a firm sense of how she would slot into scenes. Replacing Berywn, a character with much more presence, just makes her inadequacy stand out more.
B should have gone with Zo. A mentor-type character for the PC is a strong and easily grasped concept that makes her stand out and provides a very flexible basis for expanding her content. His phoenix harpy is too much an 'I and only I think this is a really cool idea' project. It's not clear how she slots into the story, her appeal is somewhat nebulous, and even in combat it's tough to pin down a role for her. Your favorite character concept isn't necessarily the one that's best suited to the companion role.
Quint hits the right notes for relevancy and was probably envisioned as 'a male Cait'. He just flat-out falls short of that mark. In practice he always comes off more crude and catty than seductive and doesn't pull that off the way Berywn does. A brooding pretty boy Don Juan who cultivates an air of mystery needs to hit specific notes in order to work. While I do think he's salvageable I don't get the sense that Bubblelord knows how to write this type of character and that's put other people off from taking a crack at him.
Cait, Brint, Etheryn, Arona, Garret, revised Quint, Atugia/Eryka hybrid, Berwyn, and Zo would be a very solid lineup.