Well, yes, that might explain why the New Republic didn't pay much attention to the First Order... if they weren't constructing a massive superweapon capable of wiping out an entire star system at once. The scale of such a project is so mindbogglingly large, that it's impossible for it to go undetected (its footprint, so to speak, would be much, much larger than the fact that the physical weapon is just one planet in a galaxy). And once it's known, it would be extremely implausible for anyone to ignore the weapon. To borrow the North Korea analogy, it would be like if North Korea was known to be constructing a super-MIRV that could destroy multiple American cities, which could not be countered once launched and could be used more than once.
At least the Expanded Universe created the contrivance of the Maw to handwave such an outlandish notion away - and the fact that it did so tells us that the Star Wars universe wasn't always so willing to blithely ignore realism to such a degree. Actually, I think it's still possible to go with the debut of Starkiller base as it is if they had actually indicated that few people in the New Republic believed it would work. That wouldn't even have been that difficult to set up, but the writers didn't think of that?
Not that this problem broke the entire movie, but it really doesn't currently make any sense.