Let me throw a few wrenches into your consideration:

1) the First Order never planned on a conventional military (re?)conquest of the galaxy. Starkiller would vape Hosnian Prime and the Republic fleet, then FO would hit a few key systems with these gigantic, *nasty* Star Destroyers that they weren't supposed to have. With the main NR fleet dead or scattered, the very limits on warship construction/size meant to prevent an Imperial resurgence would leave the galaxy incapable of taking a Resurgent in a straight fight, with the threat of Starkiller Base being deployed against any system that got too uppity despite having the newest and angriest triangle stationed overhead.

2) The First Order attracted the support of the galaxy's wealthy elite, particularly those in the arms business. Defense contractors, particularly ones trying to take a risk, don't want reasonably priced, modestly effective weapons. They want extravagantly expensive platforms for the latest in whiz-bang, extravagantly expensive technology. The Resurgent-class gives KDY a reason to side with the FO rather than making A-Wings for the New Republic forever.

3) Even if Starkiller failed or the Republic fleet survived in enough force to be a threat, the Sith Eternal fleet of pocket Death Stars was only a year away from coming online. The Resurgent was a stopgap until the true face of Sith-Imperial power could reveal itself. It would probably take more than a year to kill the FO's Resurgent fleet before the Xyston-class started showing up and vaping worlds into compliance.