This ignores, indeed craps on, the memory of Martin Luther King and the Abolitionists before him.
Are you suggesting that the only way to honor the memory of Martin Luther King and the abolitionists is to ignore, or perhaps conveniently revise, history? My precise words were "in a great many cases," and you cannot ignore the fact that "aggressive, scripturally based arguments to the contrary" were indeed mounted in connection with, for example, the institution of slavery and the extension of voting rights to women. These arguments were not instantly dismissed or easily rebutted.
You can pretend that such arguments were not made, just as you can pretend that the institution of slavery was swiftly and painlessly abolished as patently anti-Christian, but that would be, of course, nothing more than convenient fiction.
The Biblical arguments of the abolitionists were mounted in large part as a direct response to easily articulated (if patently superficial) Biblical pro-slavery arguments. The abolitionists arguments are to this day a case study in hairsplitting apologetics and hermeneutics.
Indeed, I think it is a fair argument that social norms derived from non-Biblical concepts of justice and equality informed and shaped the carefully constructed Biblical interpretations employed by the abolitionists much more than did explicit Biblical language.