I think this overestimates the power and scope of the First Order. Of course, we'll learn more in December with The Last Jedi, but as of now, the First Order is more of an insurgency coming from the outer regions. It had Starkiller, yes, but we don't know how powerful it is, but we do know it doesn't have the might to directly invade the New Republic. Most of the galaxy still lives free under post-Empire government. The conflict we're watching in The Force Awakens, is off in the wilderness somewhere and quite small. That's why Leia launched the Resistance, because the New Republic didn't see the First Order as enough of a threat to bother with. The destruction of Hosnian Prime changes that, but it was still more of a terrorist attack than a full on invasion.