I think your basic premise is flawed. I'll assume that the king you refer to is Holy Roman Emperor Henry IV and the pope you refer to is Gregory VII. While it is true that Henry made the famous "walk to Canossa" to beg the pontiff's forgiveness, that's not where the story ends. Henry was forgiven by the pope, and promptly returned home to assert his control over his vassals that had taken the pope's side. When Henry and Gregory came into conflict later and the pope excommunicated the emperor, the pope found himself without sufficient allies. Henry was able to depose and unseat Gregory, who died in exile. So in the story of just one pope and one king you have many inflection points regarding their power. Reams of paper have been written on the Investiture Controversy, which is one of many such inflection points.

Overall, I believe you overstate the power of the papacy in the past and underestimate its power in the present. Despite losing its temporal power and not calling crusades, the modern papacy has a great deal of influence. For instance, the papacy has near total control over who is named as a bishop in nearly every country on the planet. This is control that Gregory VII didn't dream of having--and a power he quite possibly would have opposed himself. Despite the Vatican's strict neutrality in foreign affairs mandated by the 1929 Lateran Accords (another inflection point, along with the earlier wars of Italian Unification), that tiny city-state has a great deal of influence in diplomacy. Further, the pope is still the head of the Roman Catholic Church, which is a far cry from being "all-show."

If you have specific questions, feel free to ask them, as I applaud an earnest search for understanding. If instead they require adhering to your interpretation of what the papacy is currently I will likely not be interested in answering them.