I feel that serving a single-page app with a catch-all fallback is fine for an app behind a login but costly for anything else you want found. Search engines will treat missing sitemap.xml/robots.txt or even llms.txt as soft 404s. Any tools that just check the status will think those files exist. So AI crawlers will either have to start using JS, this as they admit will hurt SEO, but more importantly today GEO.