Mostly a question of addition, not replacement
Large Snowflake customers often already run Tableau Cloud or Tableau Server with years of published content. When Snowflake promotes Cortex, the evaluation is less "which one" and more "does Cortex earn a place beside Tableau, and does it take any load off our analysts?"
Tableau's position rests on visualization depth. Analysts can build highly customized visual analytics that most tools cannot match, the governance and admin controls are mature, and procurement and security teams know the product well. It runs in the cloud or on self-managed servers. Its AI features are growing, generally layered into existing BI workflows. The tradeoff is operational: without dedicated BI capacity, building and maintaining content is slow, and business users depend on the people who know the tool.