Why's that company so big? I could do that in a weekend
I often hear this logic from inexperienced engineers. 
My answer to them is: Then why don't you do it? Recently a junior engineer said this to me about Uber, and I helped him to understand - His logic was "it's just a taxi app".There were loads of Taxi apps before Uber and loads of them after Uber. Yet none of them were uber?Sure that a medium complexity CRUD app can do the pickup and scheduling, and spit out some json to the app.But what if you have 100,000 journeys taking place simultaneously, and you must track their positions so that you can render the journey map on the invoice?Can you do live traffic planning and re-route around the busy traffic on the fly?Does your weekend include the full billing system? This is a mandatory requirement for a business.Can you spot busy periods and increase the fares according to the laws of supply and demand? All online in realtime? Can you continuously A/B test this to optimise profits?Can you deal with the fact that maps and roads are always changing?All of these and more are the things that make Uber, Uber, otherwise you're just one of those shitty taxi apps that failed.Still think you can build it in a weekend?