I can think of a thousand better reasons not to own a person aside from increased productivity.
The Caucasian Invasion Of Racial Justice Spaces
The DiDi Delgado

Just because you can think of additional reasons does not mean they were wrong. Believe it or not, there can be multiple arguments for a particular social order.

Also, you are completely ignoring quotes like this from the very document that you cite,

[Our ancestors’] grievances, great as they were, were trilling in comparison with the wrongs and sufferings of those for whom we plead. Our fathers were never slaves — never bought and sold like cattle — never shut out from the light of knowledge and religion — never subjected to the lash of brutal taskmasters.

Clearly, these people had more concerns than the financial well being of the slave owners.

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