I think I get it. I think the OP is not concerned with disappearance of people or anything, but whether Protestantism’s centripetal tendencies would be so strong as to cause it to devolve into ever more bizarre manifestations.
That devolution would be my view of it. Protestantism is extremely derivative. When one says one is “Protestant” one is saying, in effect, “I accept some of the tenets of historical Christianity (Catholicism) but not all. To know more, you must inquire.”
Even some Protestants will admit that their religion always has an eye on the Catholic Church, because, in Christianity, it’s the only thing that is inherently stable. Protestantism is inherently unstable, because one you take the position that personal interpretation is valid, there are no limits to where you can go with it.
In short, then, I believe Protestantism’s fragmentation would accelerate, and would devolve into ever more bizarre manifestations.