This article defines a comprehensive architectural doctrine for modern Financial Services data platforms, separating precursor decisions (what must be true for trust and scale) from foundational decisions (how the platform behaves under regulation, time, and organisational pressure). It explains why ingestion maximalism, streaming-first eventual consistency, transactional processing at the edge, domain-first design, and freshness as a business contract are non-negotiable in FS. Through detailed narrative and explicit anti-patterns, it shows how these decisions preserve optionality, enable regulatory defensibility, support diverse communities, and prevent the systemic failure modes that quietly undermine large-scale financial data platforms.