This is completely wrong. In the 1800 US census, it recorded 893,602 slaves. By the time of the 1860 census (the last before the Civil War), that had risen to 3.9 million slaves (the 1850 census showed 3.2 million, so it was obviously still rising).
And, yes, it halved as a % of the population because the urban population was growing faster than the slave population was growing. That doesn't mean it was shrinking.
Making slavery unprofitable is more impressive than actually banning it. Both are good, but enabling freedom is still a substantially more powerful change than the simply banning of slavery. Much like how banning heroin doesn't actually cause heroin to die out if there is a demand for it; we may yet see heroin legalised in my lifetime much like marijuana. It is entirely possible. Slavery ain't coming back, it benefits pretty much nobody. There isn't a billion dollar industry in slaves waiting to be discovered.