Nonsense: your "reasoning" leads (or, can lead) to some unusual places as it means that one can define "Protestant" so widely that you could fly a Unitarian Scientology Swedenborgian Mormon 747 thru it, and here would be more extreme than defining Santeria as Catholic.
Scripture refers to "Christians" as believers as described in Acts (11:26), which, by the way, hardly describes RCs, but means that definitions need to correspond to at least the basic distinctive beliefs. Chief among original Protestants was that of a exalted view of Scripture as the wholly inspired word of God, and of salvific faith as that which effected fruits of obedience and holiness, which excludes both liberals and atheists.
And those who most strongly esteem Scripture as the wholly inspired and accurate word of God are the most unified in core beliefs among religious groups.
You may want to isolate SS from the basic Scriptural interpretive hermeneutic that accompanied it, but which is also historically invalid.
The problem you need to face is that this isn't the first time Protestantism has gone down this trajectory. The Unitarians were originally Massachusetts Puritan Calvinists. Look it up. What happened? Similarly, Calvinism in its original European homes (Scotland and the Netherlands) is moribund. What happened?
If the best you can do is to try to deflect the blame on Rome, you're just fooling yourself. Something is rotten in the state of Denmark, and it isn't the Catholics' fault.