“The whole threat of slavery in the territories was just an astro turf fake concern meant to panic people into voting against the Southern states in their representation.”
That is from the point of view of the territories. To the South, it wasn’t a threat but a promise. Perhaps a fatuous one, but try telling Don Quixote that he’s fighting a windmill. It was their hope, and you’re caught up on whether that hope was realistic or not.
I'm pretty sure Southern cotton growers knew they couldn't make any money with slaves in the territories. They knew what was required in terms of land and water and labor and transport to grow and market cotton, and they certainly weren't fooled by the hype of "slavery in the territories."
And they were the dominant force in Southern politics at the time. If they allowed people to believe what they believed, it was because it suited their purpose to allow people to believe it.
This "expansion into the territories" is a straw which so many want to grasp to justify supporting the New York coalition which virtually controlled Congress at the time.
Powerful group of New York liars astroturfs the nation in an effort to retain power? Do you watch the news nowadays?