Could the incumbents simply tack on vector features? Sure, that’s the JSONB story of Postgres. It’s the regex story of all RDBMS offerings after Oracle’s acquisition of Endeca suggested a real commercial opportunity for search-specific databases.
Vector-first storage engines have a place in the market as much as the tack on solutions do. PGVector will be good enough for most users, and Weaviate (or Milvus) will be better suited for the most ambitious, or those seeking the best dev ex.