Theo migrated his T3 Chat website from Next.js to Tanstack Start because the app is highly dynamic and didn't benefit from SSR or Next.js's router, leading to a hacky React Router implementation. The decision was primarily made by his team, and the move highlights Tanstack Start's appeal to developers already familiar with the Tanstack ecosystem (Router, Query). Theo still considers Next.js the easiest way to deploy complex React apps with TypeScript backends together.