Chaosmancer
Legend
The epic spell Rain of Fire has a 2 mile radius
Fair, but I've never seen that spell and it was never mentioned. I can't even check to see how that spell is cast and if it has a cost that is high enough to make it not worthwhile. I was responding the Cloudkill's and Banshee Wail (which I can also not check, since I don't know where this spell is listed)
And those spells aren't 2 mile radi.
Okay, part of the problem here is that this is a Spelljammer discussion, which moves us to the common element of 1-3e, where SPelljammer exists. Eberron is also fundamentally and originally a 3e world.
Money and equipmentwise, eberron and the other worlds all use the same price lists, and the same wealth by level. It's all part of the rules. They have no advantage whatsoever in equipping anyone. Magewrights exist to make magitech to increase the standard of living, and have absolutely no macro influence on WBL and what can and cannot be given to your soldiers. If you are going to start claiming that Eberron can make things that Faerun cannot, the obverse also applies. Faerun has a far, far deeper magical heritage then Eberron can imagine, and again, Archcasters all over the damn place.
Look up the area of effects for Transmute Rock to Mud and Cloudkill in prior Editions. They are freaking massive, and Cloudkill moves at 30 there, not 10. If you are trying to hold a position, you are dead. It instantly kills anything of 4 HD or less, 5-6 HD saves at -4, 7-8 at normal, and 9+ are immune. So the archcaster can fill the area around him with death and be completely immune to it.
Oh, and then Widen it to double all the dimensions, too. And make it invisible, potentially.
a Lightning BOlt has a range of Long. It can rip a 10' line 1200 feet long through a formation.
Greater Lightning Storm can call down literally dozens of bolts over time onto a scattered formation, completely disrupting any unit cohesion.
Summon Monster VI+ can Summon tons of low level monsters right on top of you.
That save to avoid Wail of the Banshee is likely to be a 29. A soldier might have a 5% chance of saving against it.
A Meteor Swarm can take out half an acre, which means the core of your elite troops, or your command center, from 400 yards away or up in the air.
A Gate spell could bring in a Planetar, another 17th level caster.
Holy Word spells will instantly kill anyone of the wrong alignment and half the level or less nearby.
Those are just AoE's. An archcaster's ability to take out single, expensive targets is naturally much higher. One disintegrate spell can take out a Warforged Titan. A rain of fireballs and lightning bolts, or Just Call Lightning, from up in the sky will obliterate your artillery. Command of the weather will drown your troops in water and mud and leave them unable to go anywhere. Call in demons or the like to spread disease among your troops. Charm their commanders to give them the wrong orders.
I am not an Intellect 30 Archmage. Such beingsa re Not Dumb. Low level people have no way of dealing with everything an archcaster can bring to the table repeatedly.
The game is not the real world. In the real world, you shoot somebody, anybody, they can die instantly. In D&D, you can empty the entire clip into a high level character, and they'll just look at you funny. The combination of HP and AC is basically untouchable in the game we are talking about.
Now, if you want to go 5e... they reined in things a lot. But 5e is not the game of SPelljammer, nor the game Eberron was designed as. The bounded accuracy is not a thing. High level existences can kill armies on their own, bring down nations and kings.
It seems we have completely different premises.
I was working under the premise that if Eberron invaded a world, via some means, in 5e.
You are working under the premise of spelljamming in 3e or 2e. And, as you note, that immediately changes everything.
Wizards are immune to attacks, just period, of everyone below a threshold. At that point armies don't matter. Wizards can do anything. And at that point nothing matters. All of the stats of all of the spells are completely different, so the entire premise is shifted.
But, I will note something.
As I understand the game, the changes in the rules are canonical to the world. At least to Faerun, because of the Sundering.
That means, that while those Archcaster's may have been able to do those things a few decades ago, they no longer can. And since I do not know if they can see quantum probability, they might teleport into the middle of an army, expecting every attack from the soldiers to miss them, only to find that they don't. Smart does not mean you can't be prideful or make mistakes, it just means you make fewer of them.
Also, "Wealth by level" applies only to characters. If "Wealth by Level" applied to countries, then every Eberron nation not ruled by a being with multiple level 20 classes is broke as a joke. And magewrights existed for the purpose of giving magical gear to the soldiers, The Last War has most definetly been described to me as closer to a modern war than a medieval war. They used explosives after all. Ranged weaponry. War Magic.
So, I don't know why you would insist on declaring that "wealth by level and the phb mundane equipment chart" is all that Eberron would have access to. Eberron is wealthy, wealthy enough that they can afford to make magic items for the exclusive purpose of public health. No nation or city-state (which is another point of geopolitics) in the other countries can claim that. Despite their "deep magic" the common people see none of it. It is exclusively in the hands of the powerful. So, it is perfectly reasonable to assume the common soldier is given more magical equipment than the opposing forces, backed by the wealthy of Eberron.