What would give them they idea that all new territory would have slavery when the founding fathers outlawed slavery in the northwest territory?
I think the trouble with trying to understand the past is that there was not always a single motivation behind what was happening.
There are a lot of people who argue that slavery was on the wane, not only in the northern states, but everywhere. I think this contributed to the passage of that portion of the North West ordinance, but I think what also contributed to it was the belief that there would be no profit in slavery in any of those territories anyway.
Probably the Southern states were more accommodating of efforts to restrict or minimize slavery in those years because they saw at the time no great path to any significant profitability.
And then of course, Eli Whitney upended the whole game.
I also think some of the later concern about the "territories" had nothing to do with actually having slaves in them, but had more to do with both justifying it as acceptable and also acquiring more votes in congress.
Can congress override a constitutional right? When it didn't look like it mattered if the right was available or not, nobody cared. They started caring when they recognized it as potential future income or future political power.
In any case, an act of congress doesn't have the force or authority of an amendment, and they didn't do that, and I expect they could not have done that if they had tried.
An amendment speaks more to the will of the people than an act by their representatives and Senators.
Taney struck that restriction down as part of his Dred Scott decision.