Does EastEnders have the highest cast turnover of the big three?
Yes, for boring contractual reasons rather than creative ones!
As a BBC production (taxpayer funded) it has a fixed budget for the cast, meaning that if a character has run their course and needs a rest, they release the actor and consider bringing them back at a later date.
A show like Coronation Street doesn’t have “wasting money” restrictions so can keep a character like Ken or Rita or Audrey on a high salary and a part time contract with only a few lines each week for an indefinite period - this obviously adds to the show’s fabric and legacy but is a luxury that Eastenders does not have.