Top-down generally only works for a well-understood problem domain, and even then it only holds up for focused projects where you have the power to declare what is in scope and out of scope in a very strong fashion. This works better for dev tools, libraries, middleware or other projects that are abstract and not tied too closely to a specific business or end-user goal. In other words, the more abstract the tool and the more technical the audience, the more likely that you can drive massive impact while maintaining a simple vision and avoiding all kinds of edge cases and incidental complexity.

Rapid prototyping is more optimal for any end-user product or any new domain, because it’s a faster way to discover the unknown unknowns, both in terms of user features as well as well as technical challenges you may not have anticipated.