I think you may be underestimating the purview of this issue. Since, as Johnathan Brown in his article highlights, slavery is something that essentially transcends time and space and how we define this universal phenomenon is contingent upon our contemporary notions of freedom, the finer details can sometimes lead to discussions on legality as well as ethics and morality. For instance, if we are to regard slavery as inherently immoral for all times and places, how can we consider the Prophet (s) the manifestation of the Qur’an par excellence? Such a discussion would necessarily have to take place within the realm of philosophy.
As Sayed Ammar mentions in his lecture, slavery was an institution endorsed, not constitutionalised. It was a phenomenon so prevalent before the Prophet (s) that he saw no need to eradicate it, and any attempts to do so would have been futile anyway. It was more feasible to establish ethical guidelines to deal with slaves (see Tafsir Al-Mizan vol 12: 178-9 for examples). In any case, I think more interesting is the question that if abolition wasn’t Islam’s aim, why is slave manumission made the compensation for missing a fast, to take one example? And if abolition was never encouraged, is there ever an instance where taking in slaves was explicitly encouraged? If there wasn't it would seem as if emancipation was a practice without a long-term goal. Perhaps the Prophet (s), though it seems unlikely, thought that if emancipation occurred regularly, ‘slavery could continue to exist without being a trans-generational status, and would eventually die out’.
Similar to Jebreil's point is the one made briefly in this paper (@King you may find it useful as a critical response to Brown's paper), where slavery is looked at in relation to the cosmological divine-human relationship between us and Allah سُبْحَانَهُ وَ تَعَالَى, that we are all slaves in relation to our Lord and so our status as a slave or freeman in this transient world is of little importance.