The time warps of agile development

All trekkies have seen the shows where there's some time loop or a time fold, which results in some fantastic plot. I guess the ideas cannot be completely discarded in this amazing universe, but I don't think it applies to every day life and I don not think that agile development results in time warps. But sometimes I get the feeling that stakeholders believe that. What am I babbling about? Well, of course: changes in priority. Let say we have the following features:
A
B
C
D

And they are prioritized in that order. What happens and what is a natural part of an agile project is that story G is introduced and the stakeholders get the product owner to fit that between B and C. So what happens, well all the time spent preparing C and D becomes waste for yet some few more sprints. They might also become more complex, since story G might be something that adds complexity to the thing. So, the end result is that schedule is prolonged. It takes longer time to accomplish. The duration increases. Simple mathematics, but if you don't say that to stakeholders, it's like they think that the new priority does not affect everything else. I do wish I had Data and the crew on my team, but I don't. So, until then I'll just have to make everyone realize we're still on planet Earth.