Since pessimism is dominant, the management will probably abandon the plan.

Literal

Pessimism [subject-が] dominant [explanatory-のだから], managers [topic-は] that plan [object-を] will-abandon [probably-だろう].

悲観論が優勢 means 'pessimism is dominant.' ~のだから frames the reason as established fact. 断念する means 'to abandon, give up on.'