The property requirements might have kept slavery from being banned while it was getting started, though. As for the whole "not wanting to compete with slaves" thing, that wasn't actually a factor. As I understand it, slaves weren't looked on as competition; rather, they were the people even the poorest whites got to look down upon. No matter how low on the proverbial totem pole a white person fell, they could still feel superior to black people. To quote HPMOR:

"To sum up," Harry finished, "they don't have any power themselves. They don't have any wealth themselves. If they didn't have Muggleborns to hate, if all the Muggleborns vanished the way they say they want, they'd wake up one morning and find they had nothing. But so long as they can say purebloods are superior, they can feel superior themselves, they can feel like part of the master class. Even though your father would never dream of inviting them to dinner, even though there's not one Galleon in their vaults, even if they did worse on their OWLs than the worst Muggleborn in Hogwarts. Even if they can't cast the Patronus Charm any more. Everything is the Muggleborns' fault to them, they have someone besides themselves to blame for their own failures, and that makes them even weaker. That's what Slytherin House is becoming, pathetic, and the root of the problem is hating Muggleborns."