The mainline Protestant sects have been in decline. In the recent past, as this happened there was a rise in evangelical, non-denominational groups. They were very influential for some time, but are also now declining in numbers.
We see that there were doctrinal disputes from the earliest times. The divided parties established and maintained their positions. Jesus said His Church would be a sign of contradiction in the world. Time goes by, century after century of salvation history unfolds and His Church is always the beacon set on a hill for the world to see.
Divided Christians may engage one another in dialogue and dispute of doctrinal matters thinking that the side that presents the most convincing position will win over the opposition and in this way unity will be reestablished. I am not sure that this has ever happened.
The Nestorian heresy popped up in the early days. At the time it had a big impact on believers and the Church. The same is true of all heresies. They all had their day. There are still people in the world who hold Nestorian positions. They are almost unfindable. They are not relevant in the Church or world today. They are certainly not the beacon set on the hill. History has passed them by.
Most heresies are like this. They initially make a big noise and either fade away like the manichees, cathars, gnostics, pelagians, etc., or simply shrink to almost nothing and become irrelevant. Where are they today? They did not reunite through rapproachment and resolution of differences. They shrivelled and died like branches cut off from the vine.
Another phenomenon that happens with many heresies is they mutate. There may still be some pure Calvinists, or Lutherans, but they probably are minorities among those who bear the standards of the brands. They are irrelevant in Christianity today and minorities even among their own denominations. Maybe insignificant is a better word.
We see a few Protestant voices calling for a reembracing of the ideas of the founders of their respective religions, or even trying to identify with tradtional Catholicsm and claiming to be Catholic, their once hated nemesis, as they shrink and shrivel.
Despite their continuing insistance on the rightness of their founder's principles and claims of catholicity as they shrink before their own eyes, they persist in defending their doctrines, clinging to the liferafts, hoping that the world will wake up to realize they are right. They still insist on the truth of their founder's principles and think that if people would only realize it their denominations would be revived. No one is listening. They are fading away.
Nestorianism or Manicheism did not go away suddenly. The Manichees were strong during the time of Augustin and Aquinas was still refuting them.
Protestantism's decline was inevitable as it divided itself again and again, multiplying mutations and versions of itself, continually redefining itself. It is a disintegration.
I don't know where Protestantism will be in fifty years, but it is becoming more and more irrelevant in salvation history. Its day is past. That may not be true if you live in the Bible belt or a region where Protestantism is still a dominant force, but in the world in general it is clearly not the beacon on the hill.
We know that the Church established by Jesus was here at the beginning and she will be here til lthe end. We know what she looked like in the beginning and will just have to wait and see if she is called Lutheran, Baptist, Methodist at the end. While that sounds preposterous to me, others disagree.