The fundamental problem with your question is the perception that there are these prescribed stages of development and each stage is an advancement on the previous.

Instead, the indigenous peoples in the world were just as "advanced" as the colonizers who slaughtered and enslaved them. They were not on different stages of a tech tree like in a game, they just developed different societies.

So of course slavery was not necessary because there is no such thing as necessary advancement. Even if you argue that advancements in medicine requires more modern modes of production, places like Cuba or the Soviet Union skipped or sped through or skirted around or limitidly used Capitalism and still developed incredible health programs. So then capitalism isn't even necessary for technological advancement in that way, let alone slavery.