No need to be offended just because you face contradiction.
Concerning the Devshirme, just answer : if it would be a social advantage, why only Christian populations were concerned ? Would not it be normal that Ottomans helped in first poor muslims ?
Christians were only concerned by Devshirme because it was slavery and slavery was not legal among muslim populations.
If it was a "social help" for poor ppl, why ALL christians were concerned, even wealthy and noble families ?
But surely, rich Christians escaped from Devhsirme by corruption toward officials. There are also families who secretly exchanged their children with Muslim families, because some Muslims would have like to send their children in Janissery schools, but NOT Christians.
And why ? When you suppose that a poor family would be happy to give its children to an institution where they will become Muslims, just because financial and social advantage, you make the most serious mistakes that could be made in history : you suppose that people in 16th, 17th, 18th, and even 19th, had your own point of view about religion.

Christians believed that Muslims went to hell after their own death, because only a Christian could be saved in paradise. Muslims thought the same thing, moreover. So, of course,in Muslim mind, Devshirme was a good action : they saved Christian children from hell and offered them a promotion. But it was a great sin for Orthodox or catholic churches to give a child's soul to Satan, especially against money.
In general people don't like assimilation. Even now, if you propose to a poor Turkish family to send its children in a country where they will lost their faith and become Christians, where they will forget their origin and become greek or Armenian, be sure you'll have the same hostile reactions !
Devshirme was anti-islamic because it violated the protection of "dhimmi." And at the end it was a political mistake, because Christian populations in Balkans, who firstly appreciated the religious tolerance of Ottomans (comparing to Austrian Empire, for example), felt humiliated and above all, it was the most serious who could happened at this time : etenral damnation of children who were converted. For that reason, Bektashi order were populars, because most of tariqat sheltered "hidden christians".
Concerning the distinction you make between captivity and slavery, it is no sense. In all society, slavery institution has its own rules. In Ancient Times, for examples in Roman Empire, it was not allowed to make slave a free borned citizen, except if a man sold himself for debts. In Jewish society, it was forbidden to enslave a Jewish brother. Islam adopted the same rule, and if the Coran insists many times about the fact that it is bad to enslave a Muslim, it is because bedouin tribes fought each others and all captives could be ransomed or enslaved. The 2nd Caliph Omar, that no one could suspect to not be a sincere Muslim (except Shiites) reaffirmed clearly that slavery is not allowed among Muslims, but if a slave become Muslim, it was not an obligation to release him, it was only a pious action. So as they could not take slaves among muslims, islamic rulers encouraged slavery among captives, as Roman Empire did it also.
If islam would have firmly condemned slavery, it would have been suppressed, as the custom to kill female babies, for example. It was not the case, and slavery was general, as in China, African kingdoms, and many other civilizations.
In islamic civilization, so from the 7th until 18th century A.D, there were so 3 sources of "slaves' markets" : Wars with captive ; Turkish or African tribes who sold their own population (captives from internal war or even their own children) ; some population who were kidnapped by traders, in Eastern Europe, for example.
The condition of these slaves was, of course, very different in all the levels of the society :
You could be a military slave (mamluk), in army, or palatial service ; you could even became a vizir, and if you was tricky, you could killed your own king and became the sultan (as Ghurid dynasty in India), or Egyptian Mamluks.
You could be a Eunuch, so castrated and employed not only in harem, as much ppl supposed but also as high political functions, because as a Eunuch, you could not be tempted to betray the Sultan and founded your own dynasty. But in general, I don't suppose that most of Eunuchs were very happy to be sexually mutilated. By the way Islam was not the inventor of this practice, it was Byzantine empire who learnt to Caliphs. But it was largely used in islamic empires.
Except these cases, a slave could have the life of an ordinary servant, some of them had a "familial" status ; of course much young boys or women were also sexual servants. If we compare the condition of a slave in a rich and healthy family and the life of a poor "fellah", of course the last one was more unfortunate. But there were also many slaves who knew a life comparable to the Black Slaves in Southern States of America and French Haitian plantations : For example, in the 9th cenury A.D, the Zanj (Black African) people, who were employed in sugar-cane and irrigation plantations in Southern Iraq (Basra). Their life was so horrible that they revolted, and inspired by shi'a ideology, helped by the ground (Iraqi marshes) they succeed to resist at the Abbasid power during 15 years and weakened for a long time central power.
So slavery condition was radically different according to individual and social cases. Moreover, slavery was not an islamic institution. It was a universal fact, as so long as human economy need human workers, so until the industrial Revolution. Of course, in European Christianity, slavery was soon forbidden,but instead, serfdom was spred among peasants' class. In American and Caribean isles, economical needs were stronger than moral, and at the end, there were racial theories aiming to prove that Afro-American population were not so human than others, or that Bible did not condemn slavery (and that was true).
Progressively, machines have replaced slaves ? I don't even think so. The social situation of workers during the capitalist area (19th in Eruope and USA) was worse or equal than slavery. And even now, most of people are working against their own will or happinness, just because they have no choice. The only difference is that employers has not the obligation to shelter and give food to workers, but modern slavery is still persisting.