Aren’t the missions just another form of imperialism destroying a native culture?

It’s not clear what your definition of imperialism is. Most define imperialism as a political and economic hegemony of one nation over others.

Missions are a direct result of the mandate of Christ to his apostles and their successors to teach and to baptize all nations. Missions are the work of the Church fulfilling Christ’s command, not an attempt to achieve a political or economic hegemony.

At times governments have intruded their political or economic aims into the work of the missions. Those intrusions were unfortunate but should not stop the Church from its work. There is no doubt that when non-Christian cultures accept Christian teaching they will change. (Imagine the changes if our own culture accepted—and practiced—Christian teachings!) Cultures change but are not destroyed. That is the purpose of the missions.