Infallibility is not the same as impeccability. We do acknowledge that there have been sinful popes, yet we see that even these did not teach error. Even Peter had to be corrected by Paul in his personal conduct but we still accept his letters as the infallible word of God. Unlike how Protestant teaching has been constantly changing from the beginning, Catholic teaching has remained the same for two thousand years despite the bad popes.
NO... some Protestant churches have not been changing as you said.
Stated below is the doctrine that my "Protestant" church teaches. It is the same as it has taught since the reformation, when the Protestant church was separated from the Catholic church. And I challenge you to find one word of it that is not supported by God's word in the Holy Bible!
1. I believe in God the Father, Almighty, Maker of heaven and earth:
2. And in Jesus Christ, his only begotten Son, our Lord:
3. Who was conceived by the Holy Ghost, born of the Virgin Mary:
4. Suffered under Pontius Pilate; was crucified, dead and buried: He descended into hell:
5. The third day he rose again from the dead:
6. He ascended into heaven, and sits at the right hand of God the Father Almighty:
7. From thence he shall come to judge the quick and the dead:
8. I believe in the Holy Ghost:
9. I believe in the holy catholic church: the communion of saints:
10. The forgiveness of sins:
1l. The resurrection of the body:
12. And the life everlasting. Amen.