Some of the original 13 states had territorial claims extending to the Mississippi or even the Pacific. I don't know if Virginia, North Carolina, Georgia, Connecticut and the others would have joined the union and given up these claims if they knew that after years of paying taxes to defend, survey, settle, and organize these Western territories, that the new states they became could leave the union at will.
And cotton wasn't king yet in 1787. The lands that would later prove to be the richest and best for cotton were still unsettled, and wool and linen were more economical in most markets. It would take the invention of the cotton gin a few years later, and the mechanization of textile production to give cotton the advantage.
Virginians did have European markets for tobacco, but also many headaches. The best planters were already aware of some ills of the plantation economy: soil erosion, indebtedness, dependency on foreign markets, racial tensions, and the corruption of character among masters. So they weren't opposed to becoming part of a larger nation that would encourage industry and commerce as well as agriculture.
The same perception was as valid for far-seeing Southerners and other Americans in 1890 or 1990 as in 1790. It was only the cotton boom of the mid-19th century that deceived many in the South that sole reliance on slave-based agriculture was the way to wealth.