If someone was to put forward this argument to me in these terms, I would make two observations followed by asking two questions.
The first observation is simply this: not everyone who supported slavery was free. If we're talking about the grand and broad history of slavery, including everything from ancient Greek and Roman slavery to the slavery entrenched in the Old Testament laws, then we can be clear that this is not the case. Diocletian was the son of a slave, but did not abolish slavery upon becoming emperor of Rome. But even if we focus on the most heinous form of slavery - that which resulted in the American Civil War - there were still slaves who perpetuated slavery from positions of influence. A popular portrayal of such can be seen in the character of Stephen in the movie Django Unchained.
The second observation would be this: the support of slavery by free people does not easily equate to the support of abortion by living people. One is a statement of choice; the other a fairly meaningless tautology. In order to make the statement meaningful, I'm assuming that the premise of the first statement - something like "the class of people who support X are primarily the beneficiaries of X despite the fact that X is a detriment to another class of people" - is seeking to be applied to abortion.
If this is the case, there's two questions I would ask. The first question relates to whether the above argument actually stands on its own merits. To test this, I would offer a simple equivalence: Perhaps, if we are wanting to talk about death, we could say, "The death penalty is primarily supported by those who are unlikely to face the death penalty (and so who benefit from it as an extreme form of general and specific deterrence without facing any real personal consequences) despite the fact that the death penalty is a detriment to that class of people who commit acts the punishment for which is the death penalty." This argument can fairly easily be extended to all criminal acts and punishment for those acts. (There are both subjective and objective detriments involved in this example. So while there is a legitimate argument to be made about whether someone on the receiving end of a criminal sentence can determine for themselves what is beneficial or detrimental to themselves - which is dealt with in the second question to some degree - there are objective detriments to them that exist regardless of their opinions.)
To me, this seems like a legitimate equivalency to the argument that is being put forward in the original post. The truth is that decisions often have negative consequences for a class of people. These negative consequences can include death. If we are against a position simply because the class of people who benefit are those who support that position despite the fact that it also creates a detriment for another class of people, then we should probably stand against a great many positions. If you find the argument of the original post persuasive, then it might be worthwhile you analysing your views of various policy decisions from which you benefit which cause a detriment to others. Look out in particular for those which you don't seem to particularly benefit from, or you feel are neutral or inapplicable to you - those may be the most insidiously difficult, like the broad and immeasurable benefits of general and specific deterrence. Perhaps you're a utilitarian and you think that so long as the detriment of the other class of people is outweighed by the benefit to the first class of people, it's legitimate. I'm not going to get into an argument about all the problems I have with utilitarianism. Perhaps you will claim that the death penalty and abortion are not equivalent because aborted children are innocent. Despite the theological incongruity of that statement, an idea of justice is a legitimate question to raise, and does rather neatly segue into the second question. (That point also raises the question about those people who face the death penalty - or even who face imprisonment - who actually turned out to be innocent... but that's a discussion for another time.)
My second question is a backup to the first, assuming that the person defending the position of the original post takes a position something like, "Yes, all decisions that are supported by a class of people who benefit from that decision despite the decision being detrimental to another class of people are illegitimate. Decisions should be made using another model [that is more just]." The final clause of the position is bracketed because it's unnecessary, yet I feel it is most likely the position that people making this point are likely to take: that there is an injustice that needs to be righted through an alternate decision-making process. This leads us into the realm of the philosophical thought experiment often called the "veil of ignorance". It has existed for centuries, but a pretty popular proponent of it is John Rawls, who calls his particular model the "original position". The idea basically goes like this: that if everyone was forced into a machine that stripped them of their ability to know who they were or what characteristics they had or what class/es they were a member of in a society, and they were then asked to make decisions that would have benefits for one class and detriments for another, people would make decisions that were the least detrimental to everyone because nobody can be sure that they won't end up in the class/es for whom the decision is most detrimental.
Okay, with all that said, here's the second question: how do you overcome the issues that a veil of ignorance position poses? For instance, referring back to an aside mentioned above, how do we navigate the minefield that is subjective determination of benefit and detriment? This is even more explosive an issue for those who have no voice of their own, such as the profoundly disabled or the unborn, because now some third party must put their own subjective views forward as those that "best protect/benefit" the situation of the voiceless party. Third parties may garner no benefit from the position they support, and yet may also differ on the subjective determination of what is beneficial and what is detrimental to the voiceless party. It becomes particularly difficult if someone wants to argue that something as foundational as existence is not an objective benefit. There are lots of people who, upon weighing up their lives, may conclude that they would have preferred not to be born. This position is even taken by a few people in the Bible.
Even if everyone agrees that an objective determination of benefit and detriment in this area can be made, that doesn't get this position out of the woods. Now you are left with this problem: if there is an objective determination of benefit and detriment that can be made, why is the veil of ignorance necessary? If an objective truth can be determined, there's no need to worry about whether people benefit or detriment from the decisions that are made: the decision must be made a certain way because it is objectively right to do so, and that objective rightness can be determined and shown to be the case.
Of course anyone looking at this instantly realises that this is not how human decisions are made: people make bad or wrong decisions all the time; moreover, they even do it knowingly. And this is the final criticism about the veil of ignorance position that I will list: people take risks; people are selfish and aspirational; people are irrational. A veiled decision-making process doesn't protect decisions from that fundamental weakness.
With all the above in mind, my ultimate question would be this: how is the position in the original post helpful? What idea or concept is it putting forward that is beneficial?
