Most teams who reach real-time end up running both. Batch is the cheaper way to answer retrospective questions: weekly conversion reports, cohort cuts, lifetime value models, anything where the question is "what happened?" Real-time is for questions where the answer expires in seconds: "what should the welcome page show this person now?" or "which audience should this customer be moved into this hour?" The two systems can share the same identity graph and the same warehouse output, so running both is usually a question of cost-to-build, not architecture.