The NTF has stood off the GTVA for eighteen months at the time of the start of the game and is showing no signs of strain. Before deploying the Colossus they specifically say that the GTVA might have to seek peace terms with the NTF because the war cannot be sustained. This implies the the NTF is able to go on with an attritional combat strategy even after taking serious losses in Deneb. That means industrial production at a significant percentage of the GTVA's. Economics.
Not rly. The GTVA has to defend (or at the very least patrol) 22 systems before it fires a single shot; don't forget that they simply can't devote their entire military to the NTF war, especially after the Shivans arrive. The NTF has to only defend from 3 to 6, depending on what point in time you're focusing on.
Furthermore, it's much easier to wage a defensive war than an offensive one. A rule of thumb I've seen is that the attacking force has to be 3x the defending force to have a reasonable chance of victory. It's probably much more lopsided in the FreeSpace universe because nodes are excellent choke points.
Plunging civilization into war and economic turmoil is a pretty stupid way of distracting a civilization from economics. It's also a pretty stupid way to treat a civilization that you love. Lastly, it's quite clear that Bosch's ultimate goal was a bit more relevant to the GTVA's foreign policy than to their economic dealings.
One could argue that war in some form would have happened anyway, given the conditions at the time. Bosch stepped into the leadership void and focused the sentiments, energy, and societal current into something that he could use for his own purposes.