That's absolutely true. However, most systems that think they need scale-out are either worrying about a problem they don't have yet, suffering performance problems due to poor database design, or have not fully explored logical (dynamic or hash-based) sharding as a solution to their problems.

A huge proportion of modern software businesses can run on single-writer RDBMS instances, properly engineered, at a fraction of the operational and implementation cost of a scale-out solution. That applies to hosted and self-managed solutions equally, in my experience.