Most isekais get random power ups and brute force their way to victory. Ones that think their way through a problem, are far rarer. Parson was sold to us as a thinking man protagonist, but it's turned into a power wank that doesn't even give a power wank to the actual protagonist of the story. It's a double slap to the face.
Also +1 on casters should have solved the upkeep issue. I think the entire upkeep thing is another aspect where Rob is brute forcing a problem that shouldn't even exist in the way it does. I mean, look at FAQ. I simply refuse to believe that they can't reduce upkeep enough to be sustainable. Far more likely is that they never had an army until the prophecy was made, so they had to keep sending Jillian out on missions to make sure she wasn't around when FAQ was wiped out. That's just me inventing a solution to the problem though, which I shouldn't have to do.
That said, it was stated in a backer update that a "half" army is sustainable. So there is clearly just this need to keep popping units until you go into negative upkeep. So, if keeping an army small works, you can keep a big army of decrypted because they have 0 upkeep, it seems that solving upkeep is not a big deal for GK, or it wouldn't be, if they were not at war all the time.
Actually, maybe the main problem is that everybody is just a bunch of warmongers. Hell, even the infantry would rather go fight and die in a pointless battle then spend a long life guarding a city.