Doesn't help as much as you might think. Even if crops grow super well (say as good as modern ones with highly refined plants and really good fertilizers, which can easily be ten times or more the yield per area), you still have to actually collect them. And that's a ton of work. And then, you have to move them, and if you move the significant distances it'll get eaten in the process to fuel that movement.
And this is pretty in line with what we see. The empire
doesn't have a massive standing army. It's got a small core but the numbers come from militias/conscripted peasants. It has some big cities, but those are mostly the regional capitals or trade ports. And not modern big, 20-50k I would guess. Altdorf probably has more being both the overall capital and sitting on a trade route. Middendorf and Talabheim as former capitals too. And Nuln has the cannon factories.
First, I suggest the series of blog posts on what he calls the
Fremen mirage on A Collection of Unmitigated Pedantry. Guy is an actual history professor, and it's an entertaining read. And what I'm saying is based on that.
Now, first and this is a serious misunderstand: The advantage of an agricultural society is not really the surplus, aka how much more someone produces than they eat. I'd say that's about even. The advantage is density. You might need an acre of fields to feed a family, but you need a hundred times that much as a nomad who survives on his animals. That's why you are a nomad, because you have to keep moving to a different place for your animals to eat.
And that already is a pretty conclusive point. Even if someone is a super awesome mega warrior, they're not gonna be able to fight a hundred dudes.
Second: Casulty-averseness. Now, obviously agricultural societies are somewhat casualty averse as well. Very few people enjoy dying. But they absolutely can take it much better. For one, those peasants tend to have a bunch of kids for free labor and retirement (and also child mortality). In times of not-war, the land is kind of scarce, so a good number of those kids won't get anything and don't start families, or they have to go out and seek other forms of employment, like fighting. On the other hand, if something kills a lot of them, that frees up land and you can bounce back surprisingly fast. That's not even getting into the fact that agricultural societies often develop warrior castes, who can get killed with no problem for the food production. Either way: There is slack in the system.
As an example, let's look at the romans as mentioned in the last post, and the famous second punic war (though they had bad losses at other times too). They lost nearly 100k soldiers for basically nothing. The enemy had an army marching around their home territories. And this was Rome before it conquered anything but the boot and Sicily. You know what they did? Raised another army of 100k fought some more. Now, that whole thing stretched across a good 15 years, but that doesn't undermine the point. They could replenish even in about as bad a situation as it gets. But if you don't like Romans, you also have the black plague, which killed an absurd proportion of people (30%
on the low end) and certainly had a big impact, but also didn't really stop any country from going right on. Or the fact that "Europe at peace? Sounds fake" is pretty accurate up to around 70 years ago or so. Though it's not like Nomads don't kill raid each other all the time either.
But what about the nomads? Well, I want to point at something you said in your own post: Everyone is a warrior because of their lifestyle. And this is true, and it can be a strength. You can maximise the use of your manpower. But it also means that ever man that gets killed will directly impact your food production. And combine that with the low total population due to low density, a single defeat will tear your social structure apart. Not enough to wipe out the population, but enough that the survivors have to spend time coalescing into new groups, because the old one just lack the people to remain viable. There is no slack.
On skills, I generally agree that nomads have an advantage. I do want to point out that population density again means that even if only a fraction of the population is trained warriors, that still means they probably have more than equal numbers. Also as a side note, history is long, and medival european serf aren't the only type of peasant, roman or greek citizen farmers were a thing too.
And of course, strategic mobility is a huge advantage. I said as much in post. But one reason it was such a big deal is because it allowed them to try again and again without getting stomped the first time. But I stand by the fact that it needed the luck of aligning chances (good leader getting the right ideas and some luck during critical fights). After all: There were only ever one Mongols. But that region has been inhabited for thousands of years.
Certainly, there are more examples of nomads taking over nations. But not that many. Not if you compare it to the length of history, and the size of the world. And very often, they got kicked out again eventually. And no, there was never a cycle of nomads kicking out settled lords and going weak, before getting kicked out again. That's just flat out wrong, see the post series I liked which talks about exactly that notion.
Also rocket artillery isn't a point in favor of mongols: That's something developed and produced by settled people. Because yeah, they're better at making things. Nomads love getting shit from settled people, because turns out crafting in gener and metal working in particular are a lot easier if you can just stay in one place.
As for the application to warhammer: I refer you back to my first point. The population density of nomadic societies simply isn't enough to support those large armies, without divine intervention.