"Medium is a private company and has their right to introduce new policies, processes, and procedures." That pretty much says it all. Complaining about the owners/managers of a private company not operating the way one wants is, to invoke a literary trope, tilting at windmills.

After reading several pieces on this 'de-curation' topic, it appears that a pattern of management behavior over some years is that they don't apply feedback from any broad base of Medium writers. Also, that there is little or no transparency from management to writers.

That's just another private company doing what it does. Complaining has extremely low odds of changing behavior.

In terms of moving forward - where is the Medium replacement platform that is a writers' coop owned and managed by writers?

Because there is no way out of being governed by someone else, other than to govern oneself.