Gonna buy and play the shit out of Take Back Omega DLC, and love every second.
They don't respond because not responding harbours intrigue, mystery and discussion, which I believe was their goal with the multiple open plot threads left once the game boots you back to the Normandy. Is Shepard alive? How do people communicate and travel with no relays? What happens to all the species? Will the Reapers return if they weren't destroyed? What does synthesis entail? What happened to your crew, and how will they survive on this new planet? What's the deal with the stargazer, does he really not know about the rest of the galaxy?
It's the end of a long running trilogy set in an established universe. They wanted to end on an open, thought provoking note as it keeps their fan base guessing, theorising and speculating. They won't confirm the possibility that Shepard was indoctrinated because that's up to you to decide. It's your adventure after all!