It’s not ridiculous for Protestants to say catholics aren’t Christian. Protestant movements came about because they believed the Catholic Church wasn’t teaching the word of God. It’s not a coincidence it came about at a similar time to the printing press.
As people became more literate they started to learn the contradictions between the priests and the bible. The reaction to this resulted in movements that preached personal relationships with God and rejection of false profits (worshiping popes, saints etc).
If your not aligned with either of these religions is seems ridiculous. The reality is you can’t think Protestantism is the correct path if you believe Catholics are Christians. This is the same for many religions, they mostly claim they are right and everyone else is wrong.
I don’t think you got the central point. Protestants exist to protest Catholicism. A Protestant that recognises catholics as Christian’s wouldn’t be a Protestant.
Different sects of Protestantism are able to recognise other sects as Christian’s because a central tentant of Protestant practice is the personal relationship with God. So different sects of Protestants find that easier to accept. Where as Catholics practice a relationship with God that is tied to the organisation and the Pope.
Mormons are arguably a different religion altogether. Similar to how Jewish/Christian/Islam all share similar but different religious texts making them all different religions.
The intended outcome is othering.
You misunderstand reasonable. It does not mean good, more the natural conclusion given the context.