Argentina was considered (in some, largely British, quarters) as a rival to America and potential great power in the making until the 1890s on the back of its beef and silver exports. Australia and New Zealand flourished in great part through the sale of beef, lamb and cotton to British consumers and textile mills. Denmark, while certainly not a great power, was considered "the cream front" by its German invaders in the Second World War on the grounds of its high quality of life, supported by beef and pork industries. Russia (and Poland) were so keen to conquer Ukraine during the Russian Civil War in part due to its abundant coal and agricultural surplus (as well as its industry). South American wealth (and culture) is inextricable from beef ranching. American wealth even today is inextricable from vast mineral, hydrocarbon and agricultural resources, as well as its industrial and service sectors.
So, all of those countries were overshadowed by their more industrialized neighbors over the course of the game's time frame? That's what you examples tell me. Your examples also reinforce the need for empire; New Zealand and Australia weren't fully independent countries throughout most of the period. They were economically bound to Britain. Which, for the record, I also think the game does a fair job of mimicking.
I'm not saying agriculture wasn't important, or that mineral wealth wasn't important in the period. But what I am saying is that the big industrialized countries had all the others dancing to their tune. If you want to play in the sandbox with the big kids, you need to industrialize. You aren't going to do that with agrarianism and serfdom. (Hell, serfdom isn't even friendly to plantation style agriculture.)
The irony is that the ridiculous over-the-top industrialization we can get up to in a game of Vic3 also makes rural commodities and jobs more important over time. Due to limits on resource and agriculture buildings, we can't have a huge rural sector feeding our industries. But boy do those miners and agriculture workers make a solid living feeding my industries. They'll just never be as politically important as they might otherwise be due to those same limits. You can't have 45 million farmers in a state because, well, there aren't that many farms. But you can have 45 million factory workers in a state. (And yes, I've actually done that.)
the existing IG system and does nothing to model the changing tide of 19th century conservatism where Romantic Conservative parties
I think it can do this, but it's clunky and subject to RNG.
Like, I had Andrew Jackson become an abolitionist. But I see what you mean. In most cases, the landowners eventually become marginalized. I think that the "end of history" aspect you mention is tied to how quickly and efficiently you can industrialize. While I complain about the lack of natural resources to feed my industries, it does not escape my notice that I have cities with populations that make NYC and LA today look "meh" in terms of sheer population. I have urban/rural demographics that look like 2010 in 1890. In those situations, of course you are not going to see rural POPs outnumber those pesky socialists in the cities. Arguments about the Corn Laws? No, I don't think I've heard of such a thing. The few rural voices that supported the Corn Laws were long since drowned out by the overwhelming tide of trade unionists and industrialists. Well, that and the overwhelming number of rural workers in my colonial empire that long since flooded the market with cheap grain.