I for one haven't found any real direct Islamic justification for women wearing a niqab. Most of what I get is indirect. Anyways, it's a recent phenomenon in Somalia where Saudis took advantage from the lack of government and thus spread their backward ideology.
What does Allah do to them? The Quran is very explicit on the punishment he endows on those who break his laws. It's very gruesome. Now, why is it justice to endow an everlasting unimaginable suffering on a person who comitted a finite crime? I won't answer that for you but I have my own reasoning behind why that sort of punishment isn't justice. I mean, torture is considered wrong by pretty much every single human being but when it comes to Allah, it's somehow different?
As for the suggestion that Islam came to outlaw slavery gradually, that's nothing beside modern reinterpretation of what Islam actually intended. We know for a fact that Islam was perfected after the death of the prophet and any new addition to the religion would be recognised as biddah, the second most major sin in Islam. The abolition of slavery would be a new introduction which was not recognised by the prophet or his followers who both conducted in the practice (I should have included slavery in my list of reasons for why I left Islam). Furthermore, Allah wasn't so pleasant towards the ancient Egyptians when they supposedly enslaved the Jews for centuries according to legend. I mean, Allah gave them an ultimatum to free the Jews or else. He sent down a prophet for that one task. Allah didn't take into consideration Whether or not the manumission of such a large masses of people would negatively affect the economy of the Egyptians. It's nothing but unfounded mental gymnastics.
Furthermore, Muslims fought against the abolition of slavery. The Ottomans outlawed slavery in the 19th century on par with everyone else but the rest of the Muslim world actively fought against such human laws. In fact, unlike the west, barely any abolitionist movements have taken hold throughout the 1400 years of Islam. Slaves were treated terribly in the Muslim world and they were majority black African. Somalia is a testament to such resistance, slavery was outlawed by the colonial powers but only fully enforced during the 30s meantime Bantus were held as slaves.
The West isn't perfect and it has many wrong to account to but it is highly disingenuous to deny their single handed approach to abruptly outlaw slavery worked better than Islams supposed 1400 year gradual manumission that it only occurred due to western pressure. (Not aimed at you, I'm speaking generally).