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.