Mock system design interviewer stopped me because I never asked if the system was read-heavy or write-heavy

Jumped straight into designing a notification system, load balancer, queue, database, the whole diagram, felt confident about it. Interviewer paused me and asked what the read to write ratio looked like, and I realized I'd designed the same architecture I'd design for literally anything without checking if it even fit this use case. Redid it assuming heavy reads and the whole thing changed, caching layer moved to the front, different database choice entirely. Small question, completely changes the design, feels obvious in hindsight.