Some groups commonly classified as Protestant do indeed believe this.
I am not trying to say that every Protestant group, or even most, believe this. Only those Protestant churches which claim the others have fallen away.
One common definition, perhaps the most commonly used, for Protestant is all "Christian" groups other than Catholic, Orthodox and those groups such as Mormons and JWs which explicitly exclude themselves from the Protestant grouping.
“I am not trying to say that every Protestant group, or even most, believe this. Only those Protestant churches which claim the others have fallen away.”
By your own understanding, “Protestant” refers to a large number of groups and denominations, and so, in what you think “Protestants” believe, a very easy word - “some” placed just before “Protestants”, would make for more truthful statements as to what “Protestants” believe (or not).