I don’t see this “great fruit.” Obviously once a Protestant has undergone the paradigm shift of coming to see visible unity as important, they will realize that Protestantism has serious problems in this regard. But simply repeating a figure (by your own admission, a highly speculative figure, and one that as I have pointed out isn’t based on an actual analysis of meaningful divisions) isn’t going to do anything.
To you “doctrinal unity” means that there is an official doctrine.
To conservative Protestants, it means that there is no open dissent tolerated.
Conservative Lutherans or Presbyterians or Baptists would never put up with the level of dissent the Catholic Church does.
This is, in my opinion, very much to the Catholic Church’s credit. I do not agree with the claim that this shows disarray etc. I think it shows confidence and charity. In fact, if anything I wish the Church was even slower to discipline than it is.
But there’s no denying that individual Protestant denominations of a conservative doctrinal bent have more internal unity than the Catholic Church does.
Edwin