Seems like you were going for a bit of a twist ending where the letter is actually a threat. It didn't really land for me.

The threat could be more concise and pointed which would give it more impact. Instead of speculating about how humans may react and then saying the aliens will counter it, maybe just say something that amounts to "resistance is futile".

Its also not exactly clear to me what "the problem" is. Its that their race will someday come to an end? There is a lot to be said about this idea and I think you should explore the philosophy of that more. How does prolonging their existence save them? Seems to me like they're still left with the inevitable.