The difference is that there is unity in doctrine in the Catholic Church, simply because doctrine itself is never modified just because there are differences in opinion over it. People who feel their personal opinion is more important than the defined doctrine end by leaving the Church to follow their own opinion and are, of course, no longer Catholic. Or in some cases, they are excommunicated, if they do not leave voluntarily. In both cases, they can be reconciled, but only if they acknowledge and accept the doctrine of the Church as it is.
I think there is a problem with the use of the word “Protestantism,” because it can describe anything from orthodox Anglicans or Lutherans to fundamentally non-Christian wackos like the UCC. I’m not sure how this can be handled. I remember we once had a discussion on FR of Protestants who accept the standard Creedal definition of the Trinity as constituting the orthodox wing.
We could just call all those orthodox folks who acknowledge their salvation by the shed blood of Christ, as described in the traditional Creeds, as “Christians”.