Just in case you have any misconceptions on slavery, I'm almost 100% sure that banning slavery (during the Prophet's time) would do more bad than good.

Think about it economically, even in the US up to the 1700's over 90% of jobs were in agriculture, luckily for the US though not all of it is dry arid desert. Crop growth was a very important source of income all around the world, and as you can see this must of been drastically important in Arabia as well. However in the Arabian desert where can you grow crops as a source of income, most of the land is unusable, so income has to come from another source. 1400 years ago there wasn't huge industry, no industrial revolution etc, just crops, mining (which was not an easy to run industry without slaves) things that were mostly physical. The income then sadly has to come from something easy to come by and that is slavery.

Luckily for slaves however is that Islam opened up many opportunities for slaves, freeing slaves is a religious atonement for sins, Zakat is encouraged to be given to freeing slaves (even today, I've read an article that mentions that on average every person has 25 slaves working for them and we should be trying to free them as Muslims), beating them &/or treating them unfairly requires setting them free (He who beats a slave without cognizable offence of his or slaps him, then expiation for it is that he should set him free. - The Prophet (pbuh) / Translation of Sahih Muslim, The Book of Oaths (Kitab Al-Aiman), Book 015, Number 4079)

If slavery was abolished immediately, being the incredibly huge business it was (that other businesses e.g. mining relied on it), most likely the economy would halt causing a loss of income in the end leading to the death of thousands of Arabians including the slaves themselves.

The conditions placed by Islam allowing for a gradual change, and at minimum a better life for slaves seem to be the more moral option.