What if the Southwest Ordinance on 1790 did the same for the southwestern lands of what is today Kentucky and Tennessee?
As others have pointed out, Kentucky was never a territory, so slavery remains legal there.
For Tennessee and points south (if others are affected by similar ordinances), then slavery will just be there illegally, as it was in those parts of the Northwest Territories settled by Southerners (say hello, southern Illinois and southern Indiana).
The settlers of the southwest will consist of two groups:
(1) slaveholding Southerners; and
(2) Southerners who want to become slaveowners.
As such, slavery will be made legal twenty minutes after the new states are admitted, if it's not already made legal in their founding constitutions.
Slavery in the American Southwest is one of those things that are overdetermined by 1783. A change in the Southwest Ordinance isn't going to remove it.