Because there's too much set-up:
The conflict between Don King and Caesar over Jillian
Don's over confidence, ordering the battle to be broadcast to everyone
Parson's being reinstated as Chief Warlord as a major turning point for GK
Trammenis' overconfidence, and Don King's confidence in him
It all adds up to a big sucker punch to the gut coming up.
There has been too much
It's too obvious.
a sucker punch to who? the RCC or the Parson fanboys?
A loss here could result in the capture of Wanda, the creation of the Queen of the Undead if she croaks and auto-decrypts, or at least a major split with Stanley for using losing the army. GK is too big to be reduced to one city in quick succession, but Parson would be forced to become a field commander. Enough plot for you?
If things go badly for JS then Caesar will think that he has an opportunity to rally support among the Warlords and overthrow Don. OTOH if it goes well, Caesar must know his turns are numbered as CWL and he has to make a move soon.
Don wants Caesar to take a run at him. Why?
Just demoting or disbanding him costs loyalty of the other WLs.
He wants to see who sides with Caesar so he can deal with them too and have a completely loyal inner circle when the new heir pops.
Bunny is the key here, whichever one she sides with will come out on top.
One unrelated point. Parson will still have to account for Jillian in his plans until she passes a point of no return from Spacerock. He doesn't know for sure she won't be back until she is out of range.
I'd guess plot-wise, the right answer would be somewhere in the middle.
If Parson pulls off something brilliant enough to win the battle and thus destroy Jetstone, that leaves GK back on the unstoppable train - once Jetstone's not there to stand up to them, they can steamroll, having now decrypted another army and possibly another caster (or several, now that they're going back to the city.)
If Jetstone ends up having the clear victory they think they can have, then that ends with Wanda and Jack (and, more importantly, SCARLET) croaking, and I don't think that would be good for the story - there's a lot more to be said about the relations between decrypted and the rest.
So purely from a plot armor perspective, I'd expect something in the middle. Jetstone's going to survive, but so will Wanda and Jack. My guess is that it will be accomplished by a destruction of most-of-Wanda's-forces-but-not-Wanda, leaving them with nothing to take the garrison with. Parson's going to find a way to get Jack and Wanda to survive, but not anything more than that, and come next turn they'll run away on their dwagons. Jack might end up decrypted.
(Ooh, here's a cool possibility - Parson uses strategic intentional disbanding of dwagons, combined with a little foolamancy, to make it LOOK like Wanda croaked and her decrypted dusted. Live dragons pretend to autoengage with no leadership, get croaked and fall to the ground, after clearing out a little bit of space; one Dwagon, however, veiled as dead among all the other dead dwagons, is actually alive and hovering just over the ground (since airspace extends all the way down). Jetstone doesn't bother cleaning up the dead bodies - I mean, why bother, dead bodies automatically vanish after the turn ends, and besides, all the leadership is dead so they'd autoengage if alive. But when their turn ends, they see smoke rising from the dead bodies, they assume one of their own is burning them... the smoke turns out to be a veil for the one live dwagon, carrying Wanda and Jack, who then escape from the city. )