It might indeed be true that a person who serves society has less wisdom than a Brahmana who studies Vedas + Vedanta but that doesn't mean his nature is decided before his conduct is checked. Vidura was a Sudra and Ashwatthama a Bramhana yet we could say Vidura wasn't Tamasic and Aswatthama was Demonic. Lord Shiva does say that A Sudra if controls senses is as worshippable as a Brahmana but how can we predecide that certain group of people are Tamsik? And Sudras are supposed to work for society so they can't be lazy, idle etc. For Hindu theory of knowledge, observation and reason are the primary sources. We can observe on a daily basis Brahmins who are grossly Tamasic and Sudras that are extremely Sattvic. I did refer Prof Radhakrishnan, Gita Press, Swami Ranganathananda and Swami Mukundananda Commentaries also for these verses.
Also in Bhagavata we see verses like
Being always very clean, being free from duplicity,not stealing, always speaking the truth are natural symptoms of Sudras (SB 7.11.24)
Nonviolence, truthfulness, honesty, desire for the happiness and welfare of all others and freedom from lust, anger and greed constitute duties for all members of society.(SB 11.17.21)