I think the First Order only needs enough time to establish a proper foothold in key regions of the galaxy to hold onto their power. I mean, these planets would surrender their ships and weapons in fear of planetary destruction, then later they find out Starkiller Base is destroyed, but they’ve already surrendered their means of defense. By the time the truth is revealed, the galaxy would be at a severe disadvantage. That doesn’t even consider the “merciless legions” the First Order has to also quell opposition and strike fear in the innocent.

I elaborated on why I think that one changed sentence would add a lot to the overall narrative, but one sentence probably wouldn’t make a big difference as far as the context goes. I mean, in-universe I feel it makes logical sense that the galaxy wouldn’t immediately know that the hidden weapon that destroyed the capital actually was destroyed. And by the time they do, the First Order has already tied their hands to some extent.

I think it would be at least worth keeping in mind. IX might provide context that doesn’t really fit with this idea, or maybe it would line up well with what we see.