The obvious argument, tried and cliché as it may sound, would be that SPA are anti-web. A healthy web must be indexable and searchable. It almost seems like the rulebook now for some newcomers is to start out with SSR, make use of the auspices of the open web that allow for easy and organic discovery, - and after a significant marketshare has been captured, erecting all types of of lock-in walls, including turning SPA.