As evident by the appearance of Data's head, the changes in the timeline had already happened, It's kinda a Chicken and the Egg scenario.
Of course it is stated that the Devidians took people who were already dying of Cholera, so changes would be minimal.
The whole event was a predestination paradox. The events had already happened which is evidenced by Data's head in the cave in the 24th century. The discovery of the head lead the crew to Devidia where they ended up going back in time. If Data's head was there then that means any events that transpired in the 19th century from the interactions of those in the 24th century had already become part of the timeline before the crew went back in time also meaning that all the deaths caused by the Devidians had been part of the timeline leaving nothing to be changed the only thing that happened was the crew keeping history the way it should be.
That, or, even given the number of people who disappeared in the "Prime" timeline, as opposed to the original, none of these, or their progeny, had any significant effect on essential events.