Instead of focusing on scalability, focus on productivity. Scaling is optimization, and optimization needs to be guided by profiling or generally instrumentation in production. Having good culture and a productive infrastructure, a company can rapidly scale its systems if needed. Without such productive environment, a scalable system could crumble any time. Reflecting upon my days in Netflix, I realized that we didn't build scalability from day 1. Instead, we evolved a scalable Netflix quite easily. Almost everything was a "non-event". When needed, things just appeared to have happened. That's the magic of productivity, not scalability.
